
%\VignetteEngine{knitr::knitr}
%\VignetteIndexEntry{mlt}
%\VignetteDepends{variables, basefun, mlt, survival, eha, prodlim, truncreg, lattice, gridExtra, MASS, nnet, HSAUR3, sandwich, flexsurv, grid, latticeExtra, colorspace, multcomp, mgcv, coxinterval, ICsurv, polspline, logspline, bshazard, gss, MIICD, TransModel, dynsurv, timereg, survivalMPL, np, gamlss, Matrix, muhaz, ICE, ordinal, VGAM, AER, coin, gamlss.data, mlbench}

\documentclass[article,nojss,shortnames]{jss}

%% packages
\usepackage{thumbpdf}
\usepackage{amsfonts,amstext,amsmath,amssymb,amsthm}
\usepackage{accents}
\usepackage{color}
\usepackage{rotating}
\usepackage{verbatim}
\usepackage[utf8]{inputenc}
%% need no \usepackage{Sweave.sty}
%%\usepackage[nolists]{endfloat}

\newcommand{\cmd}[1]{\texttt{#1()}}


\usepackage{tikz}
\usetikzlibrary{shapes,arrows,chains}
\usepackage{verbatim}

<<mlt-setup, echo = FALSE, results = "hide", message = FALSE, warning = FALSE>>=
set.seed(290875)

sapply(c("mlt", "survival", "eha", "prodlim", "truncreg", "lattice", "gridExtra",
         "MASS", "nnet", "HSAUR3", "sandwich", "flexsurv", "grid", "latticeExtra", 
         "colorspace", "multcomp"), library, char = TRUE)

if (!file.exists("analysis/DVC.rda")) {
    download.file("https://zenodo.org/record/17179/files/DVC.tgz", "DVC.tgz")
    untar("DVC.tgz", file = "analysis/DVC.rda")
}
load("analysis/DVC.rda")
dvc <- c(margin.table(obs[,,"wild"], 2))

logLik.phreg <- function(object) {
    ret <- object$loglik[2]
    attr(ret, "df") <- length(coef(object))
    class(ret) <- "logLik"
    ret
}
vcov.phreg <- function(object) object$var

trellis.par.set(list(plot.symbol = list(col=1,pch=20, cex=0.7),
                     box.rectangle = list(col=1),
                     box.umbrella = list(lty=1, col=1),
                     strip.background = list(col = "white")))
ltheme <- canonical.theme(color = FALSE)     ## in-built B&W theme
ltheme$strip.background$col <- "transparent" ## change strip bg
lattice.options(default.theme = ltheme)

knitr::opts_chunk$set(echo = TRUE, results = 'markup', error = FALSE,
                      warning = FALSE, message = FALSE,
                      tidy = FALSE, cache = FALSE, size = "small",
                      fig.width = 6, fig.height = 4, fig.align = "center",
                      out.width = NULL, ###'.6\\linewidth', 
                      out.height = NULL,
                      fig.scap = NA)
knitr::render_sweave()  # use Sweave environments
knitr::set_header(highlight = '')  # do not \usepackage{Sweave}
## R settings
options(prompt = "R> ", continue = "+  ", useFancyQuotes = FALSE)  # JSS style
options(width = 75)

RC <- function(...) {
    ret <- do.call("cbind", list(...))
    mlt <- which(colnames(ret) == "mlt")
    for (i in (1:ncol(ret))[-mlt]) {
        nm <- colnames(ret)[i]
        ret <- cbind(ret, (ret[,i] - ret[,mlt]) / ret[,mlt])
        colnames(ret)[ncol(ret)] <- paste("(", nm, " - mlt)/mlt", sep = "")
    }
    print(ret, digits = 5)
    return(invisible(ret))
}
@

\newcommand{\TODO}[1]{{\color{red} #1}}

\newcommand\Torsten[1]{{\color{blue}Torsten: ``#1''}}

% File with math commands etc.
\input{defs.tex}

\renewcommand{\thefootnote}{}

%% code commands
\newcommand{\Rclass}[1]{`\code{#1}'}
%% JSS
\author{Torsten Hothorn \\ Universit\"at Z\"urich}
\Plainauthor{Hothorn}

\title{Most Likely Transformations: The \pkg{mlt} Package}
\Plaintitle{Most Likely Transformations: The mlt Package}
\Shorttitle{The \pkg{mlt} Package}

\Abstract{ The \pkg{mlt} package implements maximum likelihood estimation in
the class of conditional transformation models.  Based on a suitable
explicit parameterisation of the unconditional or conditional transformation
function using infrastructure from package \pkg{basefun}, we show how one
can define, estimate, and compare a cascade of increasingly complex
transformation models in the maximum likelihood framework.  Models for the
unconditional or conditional distribution function of any univariate
response variable are set-up and estimated in the same computational
framework simply by choosing an appropriate transformation function and
parameterisation thereof.  As it is computationally cheap to evaluate the
distribution function, models can be estimated by maximisation of the exact
likelihood, especially in the presence of random censoring or truncation. 
The relatively dense high-level implementation in the \proglang{R} system
for statistical computing allows generalisation of many established
implementations of linear transformation models, such as the Cox model or
other parametric models for the analysis of survival or ordered categorical
data, to the more complex situations illustrated in this paper.  }

\Keywords{transformation model, transformation analysis, distribution regression, conditional
distribution function, conditional quantile function, censoring,
truncation}
\Plainkeywords{transformation model, transformation analysis, distribution regression, conditional
distribution function, conditional quantile function, censoring,
truncation}

\Address{
  Torsten Hothorn\\
  Institut f\"ur Epidemiologie, Biostatistik und Pr\"avention \\
  Universit\"at Z\"urich \\
  Hirschengraben 84, CH-8001 Z\"urich, Switzerland \\
  \texttt{Torsten.Hothorn@uzh.ch} \\
  \url{http://tiny.uzh.ch/bh}
}

\begin{document}

<<mlt-citation, echo = FALSE>>=
year <- substr(packageDescription("mlt.docreg")$Date, 1, 4)
version <- packageDescription("mlt.docreg")$Version
@
\footnote{Please cite this document as: Torsten Hothorn (2020)
Most Likely Transformations: The \pkg{mlt} Package. 
\emph{Journal of Statistical Software}, 92(1), 1--68,
\url{https://dx.doi.org/10.18637/jss.v092.i01}.}

% \input{todo}

\section{Introduction}

The history of statistics can be told as a story of great conceptual ideas
and contemporaneous computable approximations thereof.  As time went by, the
computationally inaccessible concept often vanished from the collective
consciousness of our profession and the approximation was taught and
understood as the real thing.  Least squares regression emerged from
Gau{\ss}' computational trick of changing Bo\v{s}covi{\'c}' absolute to squared
error and it took $200$ years for the original, and in many aspects
advantageous, concept to surface again under the name ``quantile
regression''.  This most prominent example of an idea got lost illustrates
the impact computable approximations had and still have on our understanding
of statistical methods and procedures.  In the early days of statistical
computing, implementations of such approximations were a challenge.  With
today's computing power and software infrastructure at our fingertips, our
duty shall be to go back to the original concepts and search for ways how to
reawake them for the benefit of a simpler understanding of statistical
models and concepts.

The Leitmotiv of our discipline is to foster empirical research by
providing tools for the characterisation of deterministic and random
aspects of natural phenomena through distributions estimated from
experimental or observational data.
This paper describes an attempt to understand and unify a large class of
statistical models as models for unconditional or conditional distributions.  This sounds like an
implicitness, but do we really practice (in courses on applied statistics or
while talking to our subject-matter collaborators) what we preach in a
theory course?  Let's perform a small experiment: Pick, at random, a
statistics book from your book shelf and look-up how the general linear
model is introduced. Most probably you will find something not unlike
\begin{eqnarray*}
\rY = \alpha + \tilde{\rx}^\top \shiftparm + \varepsilon, \quad \varepsilon \sim \ND(0, \sigma^2)
\end{eqnarray*}
where $\rY$ is an absolutely continuous response, $\tilde{\rx}$ a suitable
representation of a vector of explanatory variables $\rx$ (\eg contrasts
etc.), $\shiftparm$ a vector of regression coefficients, $\alpha$ an
intercept term and $\varepsilon$ a normal error term.  Model interpretation
relies on the conditional expectation $\Ex(\rY \mid \rX = \rx) = \alpha +
\tilde{\rx}^\top \shiftparm$. Many textbooks, for example \cite{Fahrmeir_Kneib_Lang_2013}, 
\emph{define} a regression model as a model for a conditional expectation $\Ex(\rY \mid \rX = \rx) = f(\rx)$
with ``regression function'' $f$ but, as we will see, understanding
regression models as models for conditional distributions
makes it easier to see the connections between many classical and novel regression approaches.
In the linear model, the intercept $\alpha$ and
the regression parameters $\shiftparm$ are estimated by minimisation of the squared error
$(\rY - \alpha - \tilde{\rx}^\top \shiftparm)^2$.  With some touch-up in
notation, the model can be equivalently written as a model for a conditional
distribution
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \rX = \rx) = \Phi\left(\frac{\ry - \alpha - \tilde{\rx}^\top \shiftparm}{\sigma}\right) 
  \text{ or } (\rY \mid \rX = \rx) \sim \ND(\alpha + \tilde{\rx}^\top\shiftparm, \sigma^2).
\end{eqnarray*}
This formulation highlights that the model is, in fact, a model for a
conditional distribution and not just a model for a conditional mean.  It
also stresses the fact that the variance $\sigma^2$ is a model parameter in
its own right.  The usual treatment of $\sigma^2$ as a nuisance parameter
only works when the likelihood is approximated by the density of the normal
distribution.  Because in real life we always observe intervals $(\ubar{\ry}, \bar{\ry}]$
and never real numbers $\ry$, the exact likelihood is, as originally
\textit{defined} by \cite{Fisher_1934}
\begin{eqnarray*}
\Prob(\ubar{\ry} < \rY \le \bar{\ry} \mid \rX = \rx) = 
\Phi\left(\frac{\bar{\ry} - \alpha - \tilde{\rx}^\top \shiftparm}{\sigma}\right) - \Phi\left(\frac{\ubar{\ry} - \alpha - \tilde{\rx}^\top \shiftparm}{\sigma}\right)
\end{eqnarray*}
which requires simultaneous optimisation of all three model parameters $\alpha$, 
$\shiftparm$ and $\sigma$ but is exact also under other forms of random censoring. 
This exact likelihood is another prominent example of its approximation (via the
log-density $(y - \alpha - \tilde{\rx}^\top \shiftparm)^2$) winning over the basic
concept, see \cite{Lindsey_1996, Lindsey_1999}. If we were going to reformulate the model
a little further to
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \rX = \rx) = \Phi(\tilde{\alpha}_1 + \tilde{\alpha}_2 \ry - \tilde{\rx}^\top \tilde{\shiftparm})
\end{eqnarray*}
with $\tilde{\alpha}_1 = -\alpha / \sigma, \tilde{\alpha}_2 = 1 / \sigma$ and $\tilde{\shiftparm} = \shiftparm / \sigma$
we see that the model is of the form
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \rX = \rx) = \pZ(\h_\rY(\ry) - \tilde{\rx}^\top \tilde{\shiftparm})
\end{eqnarray*}
with distribution function $\pZ = \Phi$ and linear transformation $\h_\rY(\ry) = \tilde{\alpha}_1 + \tilde{\alpha}_2 \ry$
such that $\Ex(\h_\rY(\rY) \mid \rX = \rx) = \tilde{\rx}^\top \shiftparm$. If we now change $\pZ$ to the distribution
function of the minimum extreme value distribution and allow a non-linear monotone transformation
$\h_\rY$ we get
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \rX = \rx) = 1 - \exp(-\exp(\h_\rY(\ry) - \tilde{\rx}^\top \tilde{\shiftparm}))
\end{eqnarray*}
which is the continuous proportional hazards, or Cox, model (typically
defined with a positive shift term).  From this point of view, the linear
and the Cox model are two instances of so-called linear transformation
models (a misleading name, because the transformation $\h_\rY$ of the
response $\rY$ is non-linear
in the latter case and only the shift term $\tilde{\rx}^\top \tilde{\shiftparm}$
is linear in $\tilde{\rx}$).  It is now also obvious that the Cox model has
nothing to do with censoring, let alone survival times $\rY > 0$.  It is a
model for the conditional distribution of a continuous response $\rY \in
\RR$ when it is appropriate to assume that the conditional hazard function
is scaled by $\exp(\tilde{\rx}^\top \tilde{\shiftparm})$.  For both the
linear and the Cox model, application of the exact likelihood allows the
models to be fitted to imprecise, or ``censored'', observations
$(\ubar{\ry}, \bar{\ry}] \in \RR$. The generality of the class of linear transformation
models comes from the ability to change $\pZ$ and $\h_\rY$ in this flexible
class of models.

The class of linear transformation models is a subclass of conditional
transformation models \citep{Hothorn_Kneib_Buehlmann_2014}.  In this latter
class, the conditional distribution function is modelled by
\begin{eqnarray}
\Prob(\rY \le \ry \mid \rX = \rx) = \pZ(\h(\ry \mid \rx)) \label{mod:ctm}
\end{eqnarray}
where the transformation function $\h$ depends on both $\ry$ and $\rx$. This
function is unknown and this document describes how one can estimate $\h$. 
Because we are interested in analysing, \ie estimating and interpreting, the
transformation function $\h$, we slightly rephrase the title ``An Analysis
of Transformations'' of the first paper on this subject \citep{BoxCox_1964}
and refer to the methods presented in this paper as \textit{transformation
analysis}.  
Different choices of $\pZ$ and different parameterisations of $\h$ in
model~(\ref{mod:ctm}) relate to specific transformation models, including the
normal linear model, proportional hazards (Cox) and proportional odds
models for continuous or ordinal responses, parametric models for survival
regression, such as the Weibull or log-normal model, distribution regression models
or survival models with time-varying effects and much more.
We describe how members of the large class of conditional transformation models can be
specified, fitted by maximising the likelihood \citep[using the estimator
developed by][]{Hothorn_Moest_Buehlmann_2017}, and analysed in \proglang{R}
using the \pkg{mlt} add-on package \citep{pkg:mlt}.  In essence, the package
is built around two important functions.  \cmd{ctm} specifies transformation
models based on basis functions implemented in the \pkg{basefun} package
\citep{pkg:basefun} for variable descriptions from the \pkg{variables}
package \citep{pkg:variables}.  These models are then fitted to data by
\cmd{mlt}.  
%This function maximises the exact or approximate log-likelihood function using the
%\cmd{auglag} and \cmd{spg} optimisers from packages \pkg{alabama}
%\citep{pkg:alabama} and \pkg{BB} \citep{Varadhan_Gilbert_2009,pkg:BB},
%respectively.



The general workflow of working with the \pkg{mlt} package and the
organisation of this document follow the general principles of model
specification, model estimation, model diagnostics and interpretation and,
finally, model inference.  The flowchart in Figure~\ref{fig:workflow} links
these conceptual steps to packages, functions and methods discussed in this
document.  We take an example-based approach for introducing models and
corresponding software infrastructure in the main document.  The underlying
theory for model formulation, parameter estimation, and model inference is
presented along the way, and we refer the reader to
\cite{Hothorn_Moest_Buehlmann_2017} for the theoretical foundations. 
Technical issues about the three packages \pkg{variables}, \pkg{basefun},
and \pkg{mlt} involved are explained in Appendices~\ref{app:variables},
\ref{app:basefun}, and \ref{app:mlt}.


\begin{figure}
\begin{tikzpicture}[%
    >=triangle 60,              % Nice arrows; your taste may be different
    start chain=going below,    % General flow is top-to-bottom
    node distance=6mm and 40mm, % Global setup of box spacing
    every join/.style={norm},   % Default linetype for connecting boxes
    ]
% ------------------------------------------------- 
% A few box styles 
% <on chain> *and* <on grid> reduce the need for manual relative
% positioning of nodes
\tikzset{
  base/.style={draw, on chain, on grid, align=center, minimum height=4ex},
  proc/.style={base, rectangle, text width=7em},
  test/.style={base, diamond, aspect=2, text width=3em},
  term/.style={proc, rounded corners},
  % coord node style is used for placing corners of connecting lines
  coord/.style={coordinate, on chain, on grid, node distance=6mm and 10mm},
  % nmark node style is used for coordinate debugging marks
  nmark/.style={draw, cyan, circle, font={\sffamily\bfseries}},
  % -------------------------------------------------
  % Connector line styles for different parts of the diagram
  norm/.style={->, draw},
  free/.style={->, draw},
  cong/.style={->, draw},
  it/.style={font={\small\itshape}}
}
\tikzstyle{line} = [draw, -latex']

% -------------------------------------------------
% Start by placing the nodes
\node [term] (pSPEC) {Specification \\ Section~\ref{sec:trafo}};
\node [term, join] (pEST) {Estimation \\ Section~\ref{sec:mlt}};
\node [term, join] (pINTER) {Diagnostics and Interpretation \\ Section~\ref{sec:predict}};
\node [term, join] (pINF) {Inference and Simulation \\ Sections~\ref{sec:asympt},
\ref{sec:sim}};

\node [proc, right=of pSPEC] (pMOD) {Transformation Models \\ \cmd{mlt::ctm}};
\node [proc, above=20mm of pMOD] (pTRAFO) {Transformations \\
Appendix~\ref{app:basefun} \\ \pkg{pkg:basefun}};
\node [proc, right=of pTRAFO] (pVAR) {Variables \\
Appendix~\ref{app:variables} \pkg{pkg:variables}};

\node [proc, right=of pEST] (pCOEF) {Likelihood \\ \cmd{mlt::mlt}};
\node [proc, join, right=of pCOEF] (pLIK) {Parameters \\ \cmd{coef}, \cmd{logLik},
\cmd{estfun}};


\node [proc, right=of pINTER] (pTA) {Transformation Analysis \\ \cmd{plot},
\cmd{predict}};

\node [proc, right=of pINF] (pAB) {Asymptotics \& Bootstrap \\
\cmd{summary}, \cmd{simulate}};

\node [test, left=of pEST] (pDATA) {Data};

%% \path (pDATA.west) to node [near start, xshift = 1em] {$y$} (pEST);

\draw [->] (pDATA.east) -- (pEST);
\draw [->] (pMOD.west) -- (pSPEC);
\draw [->] (pVAR.west) -- (pTRAFO);
\draw [->] (pTRAFO.south) -- (pMOD);
\draw [->] (pEST.east) -- (pCOEF);
\draw [->] (pINTER.east) -- (pTA);
\draw [->] (pINF.east) -- (pAB);


\end{tikzpicture}
\caption{Workflow for transformation modelling using the \pkg{mlt} package 
         and organisation of this document. \label{fig:workflow}}
\end{figure}

Before we start looking at more complex models and associated
conceptual and technical details, we illustrate the workflow by means of
an example from unconditional density estimation.

\paragraph{Density Estimation: Old Faithful Geyser Data}

The duration of eruptions and the waiting time between eruptions of the Old
Faithful geyser in the Yellowstone national park became a standard benchmark
for non-parametric density estimation \citep[the original data were given
by][]{Azzalini_Bowman_1990}.  An unconditional density estimate for the
duration of the eruptions needs to deal with censoring because exact
duration times are only available for the day time measurements.  At night
time, the observations were either left-censored (``short'' eruption),
interval-censored (``medium'' eruption) or right-censored (``long''
eruption) as explained by \cite{Azzalini_Bowman_1990}.  This fact was widely
ignored in analyses of the Old Faithful data because most non-parametric
density estimators cannot deal with censoring. The key issue is the
representation of the unknown transformation function by $\h(\ry) =
\basisy(\ry)^\top \parm$ based on suitable basis functions $\basisy$ and
parameters $\parm$. We fit these parameters $\parm$ in the transformation model
\begin{eqnarray*}
\Prob(\rY \le \ry) = \Phi(\h(\ry)) = \Phi(\basisy(\ry)^\top \parm)
\end{eqnarray*}
by maximisation of the exact likelihood as follows. After loading package
\pkg{mlt} we specify the \code{duration} variable we are interested in
<<mlt-geyser-var, echo = TRUE>>=
library("mlt")
var_d <- numeric_var("duration", support = c(1.0, 5.0), 
                     add = c(-1, 1), bounds = c(0, Inf))
@
This abstract representation refers to a positive and conceptually
continuous variable \code{duration}. We then set-up a basis function
$\basisy$ for this variable in the interval $[1, 5]$ (which can be evaluated
in the interval $[0, 6]$ as defined by the \code{add} argument), 
in our case a monotone increasing Bernstein polynomial of order eight
(details can be found in Section~\ref{subsec:uncond})
<<mlt-geyser-basis, echo = TRUE>>=
B_d <- Bernstein_basis(var = var_d, order = 8, ui = "increasing")
@
The (in our case unconditional) transformation model is now fully defined by
the parameterisation $\h(\ry) = \basisy(\ry)^\top \parm$ and $\pZ = \Phi$
which is specified using the \cmd{ctm} function as
<<mlt-geyser-ctm, echo = TRUE>>=
ctm_d <- ctm(response = B_d, todistr = "Normal")
@
Because, in this simple case, the transformation function transforms
$\rY \sim \pY$ to $\rZ \sim \pZ = \Phi$, the latter distribution is
specified using the \code{todistr} argument.
An equidistant grid of $200$ duration times in the 
interval \code{support + add} $ = [0, 6]$ is generated by 
<<mlt-geyser-grid, echo = TRUE>>=
str(nd_d <- mkgrid(ctm_d, 200))
@
Note that the model \code{ctm_d} has no notion of the actual observations. 
The \code{support} argument of \code{numeric\_var} defines the domain 
of the Bernstein polynomial and is the only reference to the data. 
Only after the model was specified we need to
load the data frame containing the observations of \code{duration} as a
\code{Surv} object
<<mlt-geyser-data, echo = TRUE>>=
data("geyser", package = "TH.data")
head(geyser)
@
The most likely transformation (MLT) $\hat{\h}_N(\ry) = \basisy(\ry)^\top \hat{\parm}_N$ is 
now obtained from the maximum likelihood estimate $\hat{\parm}_N$ computed as
<<mlt-geyser-fit, echo = TRUE>>=
mlt_d <- mlt(ctm_d, data = geyser)
logLik(mlt_d)
@
The model is best visualised in terms of the corresponding density
$\phi(\basisy(\ry)^\top \hat{\parm}_N) \basisy^\prime(\ry)^\top \hat{\parm}_N$,
which can be extracted from the fitted model using
<<mlt-geyser-density, echo = TRUE>>=
nd_d$d <- predict(mlt_d, newdata = nd_d, type = "density")
@
The estimated density is depicted in Figure~\ref{fig:geyser-plot}.  The plot
shows the well-known bimodal distribution in a nice smooth way.  Several
things are quite unusual in this short example.  First, the model was
specified using \cmd{ctm} without reference to the actual observations, and
second, although the model is fully parametric, the resulting density
resembles the flexibility of a non-parametric density estimate (details in
Section~\ref{sec:trafo}).  Third, the exact likelihood, as defined by the
interval-censored observations, was used to obtain the model
(Section~\ref{sec:mlt}).  Fourth, inspection of the parameter estimates
$\hat{\parm}_N$ is
uninteresting, the model is better looked at by means of the estimated
distribution, density, quantile, hazard or cumulative hazard functions
(Section~\ref{sec:predict}).  Fifth, no regularisation is necessary due to
the monotonicity constraint on the estimated transformation function
(implemented as linear constraints for maximum likelihood estimation) and
thus standard likelihood asymptotics work for $\hat{\parm}_N$
(Section~\ref{sec:asympt}).  Sixth, because the model is a model for a full
distribution, we can easily draw random samples from the model and refit its
parameters using the parametric or model-based bootstrap
(Section~\ref{sec:sim}).  Seventh, all of this is not only possible
theoretically but readily implemented in package \pkg{mlt}.  The only
remaining question is ``Do all these nice properties carry over to the
conditional case, \ie to regression models?''.  The answer to this question
is ``yes!'' and the rest of this paper describes the details following the
workflow sketched in this section.  
\begin{figure}[t]
\begin{center}
<<mlt-geyser-plot, echo = FALSE>>=
plot(d ~ duration, data = nd_d, type = "l", ylab = "Density", xlab = "Duration time")
@
\caption{Old Faithful Geyser. Estimated density for duration time obtained from
         an unconditional transformation model where the transformation function
         was parameterised as a Bernstein polynomial of order eight. The plot reproduces
         Figure~1 (right panel) in \cite{Hothorn_Moest_Buehlmann_2016}. \label{fig:geyser-plot}}
\end{center}
\end{figure}

\section{Specifying Transformation Models} \label{sec:trafo}

In this section we review a cascade of increasingly complex
transformation models following \cite{Hothorn_Moest_Buehlmann_2017}
and discuss how one can specify such models using the
\cmd{ctm} function provided by the \pkg{mlt} package.  The models are fitted
via maximum likelihood by the \cmd{mlt} function (details are given in
Section~\ref{sec:mlt}) to studies from different domains.  Results obtained
from established implementations of the corresponding models in the
\proglang{R} universe are used to evaluate the implementation in package
\pkg{mlt} whereever possible.  We start with the simplest case of models for
unconditional distribution functions.

\subsection{Unconditional Transformation Models} \label{subsec:uncond}

The distribution of an at least ordered response $\rY \in \samY$ is
defined by a transformation function $\h$ and a distribution
function $\pZ$. The transformation function is parameterised in terms
of a basis function $\basisy$
\begin{eqnarray*}
\Prob(\rY \le \ry) = \pZ(\h(\ry)) = \pZ(\basisy(\ry)^\top \parm).
\end{eqnarray*}
The triple $(\pZ, \basisy, \parm)$ fully defines the distribution of $\rY$ and
is called \emph{transformation model}.
The choice of the basis function $\basisy$ depends on the measurement 
scale of $\rY$ and we can differentiate between the following situations.

\subsubsection{Discrete Models for Categorical Responses}

For ordered categorical responses $\rY$ from a
finite sample space $\samY = \{\ry_1, \dots, \ry_K\}$ the distribution function
$\pY$ is a step-function with jumps at $\ry_k$ only. We therefore
assign one parameter to each jump, \ie each element of the sample space except $\ry_K$.  This
corresponds to the basis function $\basisy(\ry_k) = \evec_{K - 1}(k)$, where
$\evec_{K-1}(k)$ is the unit vector of length $K - 1$ with its
$k$th element being one.  The transformation function $\h$ is
\begin{eqnarray*}
\h(\ry_k) & = & \evec_{K - 1}(k)^\top \parm = \eparm_k \in \RR, \quad 1 \le k < K, \quad
\text{st} \quad \eparm_1 < \dots <
\eparm_{K - 1}
\end{eqnarray*}
with $\h(\ry_K) = \infty$,  and the unconditional distribution function of $\pY$ is
$\pY(\ry_k) = \pZ(\eparm_k)$. Note that monotonicity of $\h$ is guaranteed by the
$K - 2$ linear constraints $\eparm_2 - \eparm_1 > 0, \dots, \eparm_{K -1} -
\eparm_{K -2} > 0$ when constrained optimisation is performed. The density of
a nominal variable $\rY$ can be estimated in the very same way because it is
invariant with respect to the ordering of the levels (see Section~\ref{sec:mlt}).

\paragraph{Categorical Data Analysis: Chinese Health and Family Life Survey}

The Chinese Health and Family Life Survey \citep{Parish}, conducted 1999--2000 as a
collaborative research project of the Universities of Chicago, Beijing, and
North Carolina, sampled $60$ villages and urban neighbourhoods in China. 
Eighty-three individuals were chosen at random for each location from
official registers of adults aged between $20$ and $64$ years to target a
sample of $5000$ individuals in total.  Here, we restrict our attention to
women with current male partners for whom no information was missing,
leading to a sample of $\Sexpr{nrow(CHFLS)}$ women with the following
variables: \code{R\_edu} (level of education of the responding woman),
\code{R\_income} (monthly income in Yuan of the responding woman),
\code{R\_health} (health status of the responding woman in the last year) and
\code{R\_happy} (how happy was the responding woman in the last year).
We first estimate the unconditional distribution of happiness using a
proportional odds model \citep[\cmd{polr} from package \pkg{MASS},][]{pkg:MASS}
<<mlt-CHFLS-1>>=
data("CHFLS", package = "HSAUR3")
polr_CHFLS_1 <- polr(R_happy ~ 1, data = CHFLS)
@
The basis function introduced above corresponds to a model matrix for
the ordered factor \code{R\_happy} with treatment contrasts using the largest level
(``very happy'') as baseline group. In addition, the parameters must satisfy
the linear constraint $\mC \parm \ge \mvec$, $\mC$ (argument \code{ui}) 
being the difference matrix and $\mvec = \bold{0}$ (argument \code{ci})
<<mlt-CHFLS-1-basefun>>=
nl <- nlevels(CHFLS$R_happy)
b_happy <- as.basis(~ R_happy, data = CHFLS, remove_intercept = TRUE,
                    contrasts.arg = list(R_happy = function(n) 
                        contr.treatment(n, base = nl)),
                    ui = diff(diag(nl - 1)), ci = rep(0, nl - 2))
@
A short-cut for ordered factors avoids this rather complex definition of the basis function
<<mlt-CHFLS-1-basefun-basis>>=
b_happy <- as.basis(CHFLS$R_happy)
@
We are now ready to set-up the (unconditional) transformation model by a call to
\cmd{ctm} using the basis function and a character defining the standard 
logistic distribution function for $\pZ$
<<mlt-CHFLS-1-ctm>>=
ctm_CHFLS_1 <- ctm(response = b_happy, todist = "Logistic")
@
Note that the choice of $\pZ$ is completely arbitrary as the estimated distribution
function is invariant with respect to $\pZ$.
The model is fitted by calling the \cmd{mlt} function with arguments \code{model} 
and \code{data}
<<mlt-CHFLS-1-mlt>>=
mlt_CHFLS_1 <- mlt(model = ctm_CHFLS_1, data = CHFLS)
@
The results are equivalent to the results obtained from \cmd{polr}. The
helper function \cmd{RC} prints its arguments along with the relative change 
to the \code{mlt} argument
<<mlt-CHFLS-1-cmpr>>=
logLik(polr_CHFLS_1)
logLik(mlt_CHFLS_1)
RC(polr = polr_CHFLS_1$zeta, mlt = coef(mlt_CHFLS_1))
@
Of course, the above exercise is an extremely cumbersome way
of estimating a discrete density whose maximum likelihood estimator is
a simple proportion but, as we will see in the rest of this paper, a generalisation
to the conditional case strongly relies on this parameterisation
<<mlt-CHFLS-1-pred>>=
RC(polr = predict(polr_CHFLS_1, newdata = data.frame(1), type = "prob"),
   mlt = c(predict(mlt_CHFLS_1, newdata = data.frame(1), 
                   type = "density", q = mkgrid(b_happy)[[1]])),
   ML = xtabs(~ R_happy, data = CHFLS) / nrow(CHFLS))
@

\subsubsection{Continuous Models for Continuous Responses}

For continuous responses $\rY \in \samY \subseteq \RR$ the
parameterisation $\h(\ry) = \basisy(\ry)^\top \parm$ should be smooth in $\ry$, so any polynomial or spline basis is a
suitable choice for $\basisy$.  We apply Bernstein polynomials \citep[for an
overview see][]{Farouki_2012} of order $M$
(with $M + 1$ parameters) defined on the interval $[\ubar{\imath}, \bar{\imath}]$ with
\begin{eqnarray*}
\bern{M}(\ry) & = & (M + 1)^{-1}(f_{\text{Be}(1, M + 1)}(\tilde{\ry}), \dots,
                            f_{\text{Be}(m, M - m + 1)}(\tilde{\ry}), \dots,
                            f_{\text{Be}(M + 1, 1)}(\tilde{\ry}))^\top \in \RR^{M + 1} \\
\h(\ry) & = & \bern{M}(\ry)^\top \parm =
              \sum_{m = 0}^{M} \eparm_m f_{\text{Be}(m + 1, M - m + 1)}(\tilde{\ry}) / (M + 1) \\
\h^\prime(\ry) & = & \bern{M}^\prime(\ry)^\top \parm =
              \sum_{m = 0}^{M - 1} (\eparm_{m + 1} - \eparm_m) f_{\text{Be}(m + 1, M - m)}(\tilde{\ry}) M /
((M + 1) (\bar{\imath} - \ubar{\imath}))
\end{eqnarray*}
where $\tilde{\ry} = (\ry -\ubar{\imath}) / (\bar{\imath} - \ubar{\imath}) \in [0,
1]$ and $f_{\text{Be}(m, M)}$ is the density of the Beta distribution with
parameters $m$ and $M$.  This choice is computationally attractive because
strict monotonicity can be formulated as a set of $M$ linear constraints on the
parameters $\eparm_m < \eparm_{m + 1}$ for all $m = 0, \dots, M$
\citep{Curtis_Ghosh_2011}.  Therefore, application of constrained optimisation guarantees
monotone estimates $\hat{\h}_N$. The basis contains an intercept.  


\paragraph{Density Estimation: Geyser Data (Cont'd)}
We continue the analysis of the Old Faithful data by estimating the unconditional distribution of
waiting times, a positive variable whose abstract representation can be used to generate
an equidistant grid of $100$ values
<<mlt-geyser-w>>=
var_w <- numeric_var("waiting", support = c(40.0, 100), add = c(-5, 15), 
                     bounds = c(0, Inf))
c(sapply(nd_w <- mkgrid(var_w, 100), range))
@
A monotone increasing Bernstein polynomial of order eight for \code{waiting} in the interval 
$[\ubar{\imath}, \bar{\imath}]$ \code{ = support(var_w)} is defined as
<<mlt-geyser-bernstein>>=
B_w <- Bernstein_basis(var_w, order = 8, ui = "increasing")
@
The (here again unconditional) transformation model is now fully described by
the parameterisation $\h(\ry) = \basisy(\ry)^\top \parm$ and $\pZ = \Phi$, the latter 
choice again being not important (for larger values of $M$)
<<mlt-geyser-w-ctm, echo = TRUE>>=
ctm_w <- ctm(response = B_w, todistr = "Normal")
@
The most likely transformation $\hat{\h}_N(\ry) = \basisy(\ry)^\top \hat{\parm}_N$ is 
now obtained from the maximum likelihood estimate $\hat{\parm}_N$ computed as
<<mlt-geyser-w-fit, echo = TRUE>>=
mlt_w <- mlt(ctm_w, data = geyser)
@
and we compare the estimated distribution function 
<<mlt-geyser-w-distribution, echo = TRUE>>=
nd_w$d <- predict(mlt_w, newdata = nd_w, type = "distribution")
@
with the empirical cumulative distribution function (the non-parametric maximum likelihood estimator) 
in the left panel of Figure~\ref{fig:geyser-w-plot}.

\begin{figure}[t]
\begin{center}
<<mlt-geyser-w-plot, echo = FALSE>>=
layout(matrix(1:2, ncol = 2))
plot(ecdf(geyser$waiting), col = "grey", xlab = "Waiting times", ylab = "Distribution", 
     main = "", cex = .75)
lines(nd_w$waiting, nd_w$d)
B_w_40 <- Bernstein_basis(order = 40, var = var_w, ui = "incre")
ctm_w_40 <- ctm(B_w_40, todistr = "Normal")
mlt_w_40 <- mlt(ctm_w_40, data = geyser)
nd_w$d2 <- predict(mlt_w_40, q = nd_w$waiting, type = "distribution")
lines(nd_w$waiting, nd_w$d2, lty = 2)
legend("bottomright", lty = 1:2, legend = c("M = 8", "M = 40"), bty = "n")
plot(nd_w$waiting, predict(mlt_w, q = nd_w$waiting, type = "density"), type = "l",
     ylim = c(0, .04), xlab = "Waiting times", ylab = "Density")
lines(nd_w$waiting, predict(mlt_w_40, q = nd_w$waiting, type = "density"), lty = 2)
rug(geyser$waiting, col = rgb(.1, .1, .1, .1))
@
\caption{Old Faithful Geyser. Estimated distribution (left, also featuring the empirical
         cumulative distribution function as grey circles) and density (right) function for 
         two transformation models parameterised in terms of Bernstein polynomials
         of order eight and $40$.  The right plot reproduces the density for $M = 8$ shown in
         Figure~1 (left panel) in \cite{Hothorn_Moest_Buehlmann_2016}. \label{fig:geyser-w-plot}}
\end{center}
\end{figure}

The question arises how the degree of the polynomial affects the estimated
distribution function.  On the one hand, the model $(\Phi, \bern{1}, \parm)$ only allows
linear transformation functions of a standard normal and $\pY$ is restricted to the
normal family.  On the other hand, $(\Phi, \bern{N - 1}, \parm)$ has one
parameter for each observation and $\hatpY$ is the non-parametric maximum
likelihood estimator $\text{ECDF}$ which, by the Glivenko-Cantelli lemma,
converges to $\pY$.  In this sense, we cannot choose $M$ ``too large''. This
is a consequence of the monotonicity constraint on the estimator
$\basisy^\top \hat{\parm}_N$ which, in this extreme case, just interpolates
the step-function $\pZ^{-1} \circ \text{ECDF}$. The practical effect can be inspected
in Figure~\ref{fig:geyser-w-plot} where two Bernstein polynomials of order $M = 8$ and
$M = 40$ are compared on the scale of the distribution function (left panel) and density
function (right panel). It is hardly possible to notice the difference in probabilities
but the more flexible model features a more erratic density estimate, but not overly so.
The subjective recommendation of choosing $M$ between $5$ and $10$ is
based on the quite remarkable flexibility of distributions in this range and
still managable computing times for the corresponding maximum-likelihood estimator.
%% The corresponding AIC values are $\Sexpr{round(AIC(mlt_w),1)}$ 
%% for $M = 8$ and $\Sexpr{round(AIC(mlt_w_40),1)}$ for $M = 40$ favouring the smaller model.


\subsubsection{Continuous Models for Discrete Responses}

Although a model for $\rY$ can assume an absolutely continuous distribution,
observations from such a model will always be discrete. This fact is taken 
into account by the exact likelihood (Section~\ref{sec:mlt}). In some cases, 
for example for count data with potentially large number of counts, one might
use a continuous parameterisation of the transformation function
$\bern{M}(\ry)^\top \parm$
which is evaluated at the observed counts only as in the next example.

\paragraph{Analysis of Count Data: Deer-vehicle Collisions}

\cite{Hothorn_Mueller_Held_2015} analyse roe deer-vehicle collisions reported to the police
in Bavaria, Germany, between 2002-01-01 and 2011-12-31. The daily counts range between
$\Sexpr{min(dvc)}$ and $\Sexpr{max(dvc)}$. A model for the daily number of roe deer-vehicle collision
using a Bernstein polynomial of order six as basis function is fitted using
<<mlt-dvc>>=
var_dvc <- numeric_var("dvc", support = min(dvc):max(dvc))
B_dvc <- Bernstein_basis(var_dvc, order = 6, ui = "increasing")
dvc_mlt <- mlt(ctm(B_dvc), data = data.frame(dvc = dvc))
@
The discrete unconditional distribution function (evaluated for all integers between
$\Sexpr{min(dvc)}$ and $\Sexpr{max(dvc)}$) 
<<mlt-dvc-predict>>=
q <- support(var_dvc)[[1]]
p <- predict(dvc_mlt, newdata = data.frame(1), q = q,
             type = "distribution")
@
along with the unconditional distribution
function of the Poisson distribution with rate estimated from the data 
are given in Figure~\ref{fig:dvc-plot}. 
The empirical cumulative distribution function is smoothly approximated
using only seven parameters.  There is clear evidence for overdispersion as
the variance of the Poisson model is much smaller than the variance
estimated by the transformation model.

\begin{figure}
\begin{center}
<<mlt-dvc-plot, echo = FALSE>>=
plot(ecdf(dvc), col = "grey", xlab = "Number of Roe Deer-Vehicle Collisions",
     ylab = "Distribution", main = "", cex = .75)
lines(q, p, col = "blue")
lines(q, ppois(q, lambda = mean(dvc)), col = "darkred")
legend(400, .3, pch = c(20, NA, NA), lty = c(NA, 1, 1), 
       legend = c("ECDF", "Transformation Model", "Poisson"), bty = "n", cex = .8,
       col = c("grey", "blue", "darkred"))
@
\caption{Deer-vehicle Collisions. Unconditional distribution 
of daily number of roe deer-vehicle collisions in Bavaria, Germany,
between 2002 and 2011. Grey circles represent the empirical cumulative
distribution function, the blue line the unconditional transformation function
and the red line the Poisson distribution fitted to the data.\label{fig:dvc-plot}}
\end{center}
\end{figure}

\subsubsection{Discrete Models for Continuous Responses}

In some applications one might not be interested in estimating the whole distribution 
function $\pY$ for a conceptually absolutely continuous response $\rY$ 
but in probabilities $\pY(\ry_k)$ for some grid $\ry_1, \dots, \ry_K$ only. The discrete 
basis function $\h(\ry_k) = \evec_{K - 1}(k)^\top \parm = \eparm_k$ can be used in this
case. For model estimation, only the discretised observations 
$\ry \in (\ry_{k - 1}, \ry_k]$ enter the likelihood.

\subsection{Linear Transformation Models} \label{subsec:ltm}

A linear transformation model features a linear shift of a typically non-linear transformation
of the response $Y$ and is the simplest form of a regression model in the class of
conditional transformation models. The conditional distribution function is
modelled by
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \rX = \rx) = \pZ(\h(\ry \mid \rx)) = 
\pZ(\h_\rY(\ry) - \tilde{\rx}^\top \shiftparm).
\end{eqnarray*}
The transformation $\h_\rY$, sometimes called ``baseline transformation'', can be parameterised
as discussed in the unconditional situation using an appropriate basis
function $\basisy$
\begin{eqnarray*}
\pZ(\h_\rY(\ry) - \tilde{\rx}^\top \shiftparm) = \pZ(\basisy(\ry)^\top \parm_1 - \tilde{\rx}^\top \shiftparm).
\end{eqnarray*}
The connection to more complex models is highlighted by the introduction of a 
basis function $\basisyx$ being conditional on the explanatory variables $\rx$, \ie
\begin{eqnarray*}
\pZ(\h(\ry \mid \rx)) = \pZ(\h_\rY(\ry) - \tilde{\rx}^\top \shiftparm) = \pZ(\basisyx(\ry, \rx)^\top \parm) = \pZ(\basisy(\ry)^\top \parm_1 - \tilde{\rx}^\top \shiftparm)
\end{eqnarray*}
with $\basisyx = (\basisy^\top, -\tilde{\rx}^\top)^\top$. 
The definition of a linear transformation model only requires the basis $\basisy$, 
the explanatory variables $\rx$ and a distribution $\pZ$. The latter choice is now important,
as additivity of the linear predictor is assumed on the scale of $\pZ$. It is convenient to
supply negative linear predictors as $\Ex(\h_\rY(\rY) \mid \rX = \rx) = \tilde{\rx}^\top \shiftparm$
(up to an additive constant $\Ex(\rZ)$).
One exception is the Cox model with positive shift and $\Ex(\h_\rY(\rY) \mid \rX = \rx) = -\tilde{\rx}^\top \shiftparm$. 
Large positive values of the linear predictor correspond to low expected survival times and thus a high risk.

\subsubsection{Discrete Responses}

\paragraph{Categorical Data Analysis: Chinese Survey (Cont'd)} We want to study the impact of
age and income on happiness in a proportional odds model, here fitted using \cmd{polr} first
<<mlt-CHFLS-2, cache = TRUE>>=
polr_CHFLS_2 <- polr(R_happy ~ R_age + R_income, data = CHFLS)
@
In order to fit this model using the \pkg{mlt} package, we need to 
set-up the basis function for a negative linear predictor without intercept 
in addition to the basis function for happiness (\code{b\_happy})
<<mlt-CHFLS-2-base>>=
b_R <- as.basis(~ R_age + R_income, data = CHFLS, remove_intercept = TRUE, 
                negative = TRUE)
@
The model is now defined by two basis functions, one of the response and one for the
shift in addition to $\pZ$ and fitted using the \cmd{mlt} function; the columns of the
model matrix are scaled to $[-1, 1]$ before fitting the parameters by \code{scale = TRUE}
<<mlt-CHFLS-2-ctm>>=
ctm_CHFLS_2 <- ctm(response = b_happy, shifting = b_R, 
                   todistr = "Logistic")
mlt_CHFLS_2 <- mlt(ctm_CHFLS_2, data = CHFLS, scale = TRUE)
@
Again, the results of \cmd{polr} and \cmd{mlt} are equivalent
<<mlt-CHFLS-2-cmpr>>=
logLik(polr_CHFLS_2)
logLik(mlt_CHFLS_2)
RC(polr = c(polr_CHFLS_2$zeta, coef(polr_CHFLS_2)), 
   mlt = coef(mlt_CHFLS_2))
@
The regression coefficients $\shiftparm$ are the log-odds ratios, \ie 
the odds-ratio between any two subsequent happiness categories is
$\Sexpr{round(exp(coef(mlt_CHFLS_2)["R_age"]), 4)}$ for each year of age and
$\Sexpr{round(exp(coef(mlt_CHFLS_2)["R_income"]), 4)}$ for each additional Yuan earned.
Therefore, there seems to be a happiness conflict between getting older \emph{and} richer.

\subsubsection{Continuous Responses}

\paragraph{Survival Analysis: German Breast Cancer Study Group-2 (GBSG-2) Trial} 

This prospective, controlled clinical trial on the treatment of node positive
breast cancer patients was conducted by the German Breast Cancer Study 
Group \citep[GBSG-2,][]{gbsg2:1994}.  Patients not older than $65$ years with
positive regional lymph nodes but no distant metastases were included in
the study.  Out of $686$ women, $246$ received hormonal therapy whereas the
control group of $440$ women did not receive hormonal therapy.  Additional
variables include age, menopausal status, tumour size, tumour grade, number of
positive lymph nodes, progesterone receptor, and estrogen receptor.  The
right-censored recurrence-free survival time is the response variable of
interest, \ie a positive absolutely continuous variable
<<mlt-GBSG2-1, echo = TRUE>>=
data("GBSG2", package = "TH.data")
GBSG2y <- numeric_var("y", support = c(100.0, max(GBSG2$time)), 
                      bounds = c(0, Inf))
GBSG2$y <- with(GBSG2, Surv(time, cens))
@
We start with the Cox model $(\pMEV, (\bern{10}^\top,
\tilde{\rx}^\top)^\top, (\parm_1^\top, \shiftparm^\top)^\top)$, in more
classical notation the model 
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \rX = \rx) = 1 - \exp(-\exp(\bern{10}(\ry)^\top \parm_1
                                                   + \tilde{\rx}^\top \shiftparm),
\end{eqnarray*}
where $\tilde{\rx}$ contains the treatment
indicator and all other explanatory variables in the transformation function
$\h(\ry \mid \rx) = \bern{10}(\ry)^\top \parm_1 + \tilde{\rx}^\top \shiftparm$
with log-cumulative baseline hazard $\h_\rY(\ry) = \bern{10}(\ry)^\top
\parm_1$
\citep[the positive shift being in line with the implementation in \cmd{coxph} in package 
\pkg{survival},][]{Therneau_Grambsch_2000,pkg:survival}
<<mlt-GBSG2-1-Cox>>=
B_GBSG2y <- Bernstein_basis(var = GBSG2y, order = 10, ui = "increasing")
fm_GBSG2 <- Surv(time, cens) ~ horTh + age + menostat + tsize + tgrade +
                               pnodes + progrec + estrec
ctm_GBSG2_1 <- ctm(B_GBSG2y, shifting = fm_GBSG2[-2L], data = GBSG2,
                   todistr = "MinExtrVal")
@
\code{fm_GBSG2[-2L]} is the right hand side of the model formula and
defines the basis function for the shift term in the classical formula language.
The distribution function $\pZ$ is the distribution function of the minimum
extreme value distribution. The specification of the right-hand side of the
formula as argument \code{shifting} along with the data (argument \code{data})
is equivalent to a basis function
<<mlt-GBSG2-1-xbasis, eval = FALSE>>=
as.basis(fm_GBSG2[-2L], data = GBSG2, remove_intercept = TRUE)
@
Note that the model matrix is set-up with intercept term first, ensuring
proper coding of contrasts. Because the intercept in this model is the
log-cumulative baseline hazard function $\h_\rY$, the intercept column is removed
from the model matrix prior to model estimation.

In contrast to the classical Cox model where only $\shiftparm$ is estimated
by the partial likelihood, we estimate all model parameters $(\parm_1^\top, \shiftparm^\top)$ simultaneously
under ten linear constraints
<<mlt-GBSG2-1-mlt>>=
mlt_GBSG2_1 <- mlt(ctm_GBSG2_1, data = GBSG2, scale = TRUE)
@
The results obtained for $\shiftparm$ from
the partial and the exact log-likelihood are practically equivalent
<<mlt-GBSG2-1-coxph>>=
coxph_GBSG2_1 <- coxph(fm_GBSG2, data = GBSG2, ties = "breslow")
cf <- coef(coxph_GBSG2_1)
RC(coxph = cf, mlt = coef(mlt_GBSG2_1)[names(cf)])
@
A practically important question is how the order $M$ of the Bernstein polynomial
affects the results. Recall that the log-cumulative baseline hazard function $\h_\rY$ is not
specified when the partial likelihood is maximised and thus \cmd{coxph} makes no
assumptions regarding this function. To study the impact of $M$ on the results, 
we refit the Cox model using \cmd{mlt} for $M = 1, \dots, 30$ and plot the 
log-cumulative hazard function $\h_\rY$ for different $M$ along with the
non-parametric Nelson-Aalen-Breslow estimator in the left panel of 
Figure~\ref{fig:GBSG2-coxph_mlt}. In the right panel of Figure~\ref{fig:GBSG2-coxph_mlt},
the change in the regression coefficients $\shiftparm$ as a function of order $M$
is shown. Both the log-cumulative hazard function and the regression coefficients
obtained from \cmd{mlt} are stable for $M \ge 10$ and very similar to the results
one obtains from \cmd{coxph}. This result shows that a simple yet fully parametric model 
produces practically equivalent results when compared to the 
semiparametric partial likelihood approach. There is no harm in choosing $M$ ``too large'',
except longer computing times of course. In this sense, there is no need to ``tune'' $M$.

\begin{figure}[t]
\begin{center}
<<mlt-GBSG2-coxph_mlt, echo = FALSE, results = "hide", cache = TRUE>>=
ndtmp <- as.data.frame(mkgrid(GBSG2y, 100))

ord <- c(1:30, 35, 40, 45, 50)
p <- vector(mode = "list", length = length(ord))
CF <- c()
ll <- numeric(length(ord))
tm <- numeric(length(ord))

for (i in 1:length(ord)) {
    print(i)
    B_GBSG2ytmp <- Bernstein_basis(var = GBSG2y, order = ord[i], ui = "increasing")
    ctmi <- ctm(B_GBSG2ytmp, shifting = fm_GBSG2[-2L], data = GBSG2,
                   todistr = "MinExtrVal")
    tm[i] <- system.time(mlti <- mlt(ctmi, data = GBSG2, scale = TRUE))[1]

    ll[i] <- logLik(mlti)
    p[[i]] <- predict(mlti, newdata = ndtmp, type = "trafo", terms = "bresponse")

    cf <- coef(mlti)
    cf <- cf[-grep("Bs", names(cf))]
    CF <- rbind(CF, cf)
}
colnames(CF) <- names(cf)

of <- model.matrix(coxph_GBSG2_1) %*% coef(coxph_GBSG2_1) 
coxphtmp <- coxph(Surv(time, cens) ~ 1  + offset(of), data = GBSG2)
b <- survfit(coxphtmp)
layout(matrix(1:2, nc = 2))
col <- rgb(.1, .1, .1, .1)
ylim <- range(unlist(p)[is.finite(unlist(p))])
plot(ndtmp$y, p[[1]], type = "l", ylim =  ylim, col = col, xlab = "Survival Time (days)",
     ylab = "Log-cumulative Hazard")
out <- sapply(p, function(y) lines(ndtmp$y, y, col = col))
lines(b$time, log(b$cumhaz), col = "red")

ylim <- range(CF)
plot(ord, CF[,1], ylim = ylim, col = col[1], xlab = "Order M",
     ylab = "Coefficients", type = "n")
for (i in 1:ncol(CF)) {
    points(ord, CF[,i], cex = .5, pch = 19)
    abline(h = coef(coxph_GBSG2_1)[i], col = "darkgrey")
}
# text(20, coef(coxph_GBSG2_1) + .1, names(coef(coxph_GBSG2_1)))
@
\caption{GBSG-2 Trial. Comparison of exact and partial likelihood for order $M = 1, \dots, 30, 35, 40, 45, 50$
         of the Bernstein polynom approximating the log-cumulative hazard function $\h_\rY$.
         In the left panel the estimated log-cumulative hazard functions for varying
         $M$ obtained by \cmd{mlt} are shown in grey and the Nelson-Aalen-Breslow estimator
         obtained from \cmd{coxph} in red. The right panel shows the trajectories of the
         regression coefficients $\shiftparm$ obtained for varying $M$ from \cmd{mlt} as
         dots. The horizonal lines represent the partial likelihood estimates from \cmd{coxph}. 
         This figure reproduces Figure~6 in \cite{{Hothorn_Moest_Buehlmann_2016}}.
         \label{fig:GBSG2-coxph_mlt}}
\end{center}
\end{figure}


A comparison with the \cite{Royston_Parmar_2002} spline model as implemented in the
\pkg{flexsurv} package \citep{pkg:flexsurv} shows that the two spline parameterisations
of the log-cumulative hazard function $\h_\rY$ are also practically equivalent (see Figure~\ref{GBSG2-1-fss-plot})
<<mlt-GBSG2-1-fss, cache = TRUE>>=
kn <- log(support(GBSG2y)$y)
fss_GBSG2_1 <- flexsurvspline(fm_GBSG2, data = GBSG2, scale = "hazard", 
                              k = 9, bknots = kn)
logLik(fss_GBSG2_1)
logLik(mlt_GBSG2_1)
cf <- coef(coxph_GBSG2_1)
RC(coxph = cf, mlt = coef(mlt_GBSG2_1)[names(cf)],
   fss = coef(fss_GBSG2_1)[names(cf)])
@

\begin{figure}
\begin{center}
<<mlt-GBSG2-1-fss-plot, echo = FALSE>>=
p1 <- summary(fss_GBSG2_1, newdata = GBSG2[1,], ci = FALSE)
p2 <- predict(mlt_GBSG2_1, newdata = GBSG2[1, all.vars(fm_GBSG2[-2L])], 
              q = p1[[1]]$time, type = "survivor")
plot(p1[[1]]$time, p1[[1]]$est, type = "l", lty = 1, xlab = "Survival Time (days)", 
     ylab = "Probability", ylim = c(0, 1))
lines(p1[[1]]$time, p2[,1], lty = 2)
p3 <- survfit(coxph_GBSG2_1, newdata = GBSG2[1,])
lines(p3$time, p3$surv, lty = 3)
legend("topright", lty = 1:3, legend = c("flexsurvspline", "mlt", "coxph"), bty = "n")
@
\caption{GBSG-2 Trial. Estimated survivor functions for patient 1  
         by the most likely transformation model (MLT) and the \cite{Royston_Parmar_2002} spline
         model fitted using \cmd{flexsurvspline} as well as based on the Nelson-Aalen-Breslow
         estimator from \cmd{coxph}. \label{GBSG2-1-fss-plot}}
\end{center}
\end{figure}


Accelerated Failure Time (AFT) models arise when one restricts the 
baseline transformation $\h_\rY(\ry)$ to a possibly scaled log-transformation. With
$\h_\rY(\ry) = \eparm_1 \log(\ry) + \eparm_2$ (with $\eparm_1 \equiv 1$) and $\pZ = \pMEV$ the exponential AFT model arises
which can be specified as the Cox model above, only the Bernstein basis for time 
needs to be replaced by a log-transformation and the corresponding parameter is restricted
to one in the estimation
<<mlt-GBSG2-2>>=
ly <- log_basis(GBSG2y, ui = "increasing")
ctm_GBSG2_2 <- ctm(ly, shifting = fm_GBSG2[-2L], data = GBSG2, 
                   negative = TRUE, todistr = "MinExtrVal")
mlt_GBSG2_2 <- mlt(ctm_GBSG2_2, data = GBSG2, fixed = c("log(y)" = 1), 
                   scale = TRUE)
@
The intercept $\eparm_2$ is contained in the log basis function \code{ly}
and not in the shift term.  The \cmd{survreg} (package \pkg{survival}) and 
\cmd{phreg} \citep[package \pkg{eha},][]{pkg:eha} functions 
fit the same model, the results are again equivalent up to the sign of the regression 
coefficients
<<mlt-GBSG2-2-exp>>=
survreg_GBSG2_2 <- survreg(fm_GBSG2, data = GBSG2, dist = "exponential")
phreg_GBSG2_2 <- phreg(fm_GBSG2, data = GBSG2, dist = "weibull", 
                       shape = 1)
logLik(survreg_GBSG2_2)
logLik(phreg_GBSG2_2)
logLik(mlt_GBSG2_2)
RC(survreg = coef(survreg_GBSG2_2)[names(cf)], 
   phreg = -coef(phreg_GBSG2_2)[names(cf)], 
   mlt = coef(mlt_GBSG2_2)[names(cf)])
@
If we allow a scaled log-transformation $\h_\rY(\ry) = \eparm_1 \log(\ry) + \eparm_2$ (the intercept
$\eparm_2$ is included in the baseline transformation), the resulting Weibull
AFT model is fitted by \cmd{mlt}, \cmd{survreg} and \cmd{phreg} using
<<mlt-GBSG2-3>>=
mlt_GBSG2_3 <- mlt(ctm_GBSG2_2, data = GBSG2, scale = TRUE)
survreg_GBSG2_3 <- survreg(fm_GBSG2, data = GBSG2, dist = "weibull")
phreg_GBSG2_3 <- phreg(fm_GBSG2, data = GBSG2, dist = "weibull")
logLik(survreg_GBSG2_3)
logLik(phreg_GBSG2_3)
logLik(mlt_GBSG2_3)
RC(survreg = coef(survreg_GBSG2_3)[names(cf)] / survreg_GBSG2_3$scale, 
   phreg = - coef(phreg_GBSG2_3)[names(cf)], 
   mlt = coef(mlt_GBSG2_3)[names(cf)])
@
The estimated scale parameters $\hat{\eparm}_1$ are $\Sexpr{round(1 / survreg_GBSG2_3$scale,
4)}$ (\cmd{survreg}), $\Sexpr{round(exp(coef(phreg_GBSG2_3)["log(shape)"]), 4)}$ 
(\cmd{phreg}) and $\Sexpr{round(coef(mlt_GBSG2_3)["log(y)"], 4)}$ (\cmd{mlt}).

It is also possible to combine the log-transformation with the Bernstein
polynomial. In this case, Bernstein basis functions are computed for log-transformed
survival times, thus a linear Bernstein polynomial is equivalent to a
Weibull model. The \code{log\_first = TRUE} argument to
\cmd{Bernstein\_basis} implements this concept; the Cox model for the GBSG2
study can be implemented as
<<mlt-GBSG2-3a>>=
log_GBSG2y <- numeric_var("y", support = c(100.0, max(GBSG2$time)), 
                          bounds = c(0.1, Inf))
lBy <- Bernstein_basis(log_GBSG2y, order = 10, ui = "increasing", 
                       log_first = TRUE)
ctm_GBSG2_3a <- ctm(lBy, shifting = fm_GBSG2[-2L], data = GBSG2, 
                   negative = FALSE, todistr = "MinExtrVal")
mlt_GBSG2_3a <- mlt(ctm_GBSG2_3a, data = GBSG2, scale = TRUE)
logLik(mlt_GBSG2_3a)
RC(coxph = cf, mlt = coef(mlt_GBSG2_3a)[names(cf)])
@


\paragraph{Non-normal Linear Regression: Boston Housing Data}

The Boston Housing data are a prominent test-bed for parametric and
non-parametric alternatives to a normal linear regression model.
Assuming a conditional normal distribution for the 
median value of owner-occupied homes (medv, in USD $1000$'s, we use the corrected version) 
in the normal linear model with constant variance
\begin{eqnarray*}
\text{medv} \mid \rX = \rx \sim \ND(\alpha + \tilde{\rx}^\top \shiftparm, \sigma^2)
\end{eqnarray*}
as implemented in \cmd{lm} is rather restrictive
<<mlt-BostonHousing-lm>>=
data("BostonHousing2", package = "mlbench")
lm_BH <- lm(cmedv ~ crim + zn + indus + chas + nox + rm + age + dis + 
            rad + tax + ptratio + b + lstat, data = BostonHousing2)
@
We relax the model formulation without sacrificing
the simplicity of a linear predictor of the explanatory variables
in the linear transformation model
\begin{eqnarray*}
\Prob(\text{medv} \le \ry \mid \rX = \rx) = \Phi(\h_\rY(\ry) - \tilde{\rx}^\top \shiftparm)
= \Phi(\bern{6}(\ry)^\top \parm_1 - \tilde{\rx}^\top \shiftparm)
\end{eqnarray*}
and estimate \emph{all} model parameters $(\parm_1, \shiftparm)$ simultaneously, \ie both the
regression coefficients \emph{and} the baseline transformation $\h_\rY$. Because it is 
straightforward to evaluate the conditional distribution function, the likelihood
can deal with right-censored \code{medvc} observations ($\ge 50$). This censoring
was mostly ignored in other parametric or non-parametric analyses of this data set.

We start with a suitable definition of median value of owner-occupied homes,
set-up a \code{Surv} object for this response and a model formula
<<mlt-BostonHousing-mlt>>=
BostonHousing2$medvc <- with(BostonHousing2, Surv(cmedv, cmedv < 50))
var_m <- numeric_var("medvc", support = c(10.0, 40.0), bounds = c(0, Inf))
fm_BH <- medvc ~ crim + zn + indus + chas + nox + rm + age + 
                 dis + rad + tax + ptratio + b + lstat
@

First, we aim at fitting a normal linear model taking censored observations
properly into account. With linear baseline transformation $\h_\rY$, \ie a
Bernstein polynomial of order one or a linear function with intercept and
positive slope, the model is equivalent to the model underlying \cmd{lm} as
explained in the introduction. Only the likelihood changes when we fit the
model via
<<mlt-BostonHousng-mlt-linear>>=
B_m <- polynomial_basis(var_m, coef = c(TRUE, TRUE), 
                        ui = matrix(c(0, 1), nrow = 1), ci = 0)
ctm_BH <- ctm(B_m, shift = fm_BH[-2L], data = BostonHousing2, 
              todistr = "Normal")
lm_BH_2 <- mlt(ctm_BH, data = BostonHousing2, scale = TRUE)
logLik(lm_BH_2)
@
%%% FIXME: survreg gives larger likelihood when `rm' in included
%%% same results without variable `rm'. No idea where the problem lies.
%% tmp <- survreg(fm_BH, data = BostonHousing2, dist = "gaussian")
%% logLik(tmp)
%% cf1 <- coef(tmp)
%% cf2 <- -cf1 / tmp$scale
%% cf2 <- c(cf2[1], 1 / tmp$scale, cf2[-1])
%% logLik(lm_BH_2, parm = cf2)
In a second step, we are interested in a possibly non-linear transformation of the response
and use a Bernstein polynomial of order six. In principle, this approach is equivalent
to using a Box-Cox transformation but with a more flexible transformation function.
In a sense, we don't need to worry too much about the error distribution $\pZ$
as only the additivity assumption on our linear predictor depends on this choice (which
may or may not be a strong assumption!).
The conditional transformation model is now given by this transformation of the response, a linear predictor
of the explanatory variables the model and the normal distribution function; the \cmd{mlt}
function fits the model to the data
<<mlt-BostonHousing-ctm>>=
B_m <- Bernstein_basis(var_m, order = 6, ui = "increasing")
ctm_BH <- ctm(B_m, shift = fm_BH[-2L], data = BostonHousing2, 
              todistr = "Normal")
mlt_BH <- mlt(ctm_BH, data = BostonHousing2, scale = TRUE)
logLik(mlt_BH)
@
The model can be compared with a normal linear model (fitted by \cmd{lm}) on the
scale of the fitted conditional distribution functions. Figure~\ref{fig:BostonHousing-plot}
shows the fitted values, \ie the linear predictor $\tilde{\rx}_i^\top \hat{\shiftparm}_N$ 
for each observation, and the observed response
overlayed with the conditional distribution function for the corresponding observations.
For the normal linear model featuring a \emph{linear} baseline transformation
$\h_\rY$, the fit seems appropriate for observations with linear predictor 
less than $30$. For larger values, the linear predictor underestimates the observations.
The conditional distribution obtained from the linear transformation model captures
observations with large values of the response better. For smaller values of the response,
the fit resembles the normal linear model, although with a smaller conditional variance.

\begin{figure}
\begin{center}
<<mlt-BostonHousing-plot, echo = FALSE, results = "hide">>=
q <- 3:52
m <- predict(lm_BH, data = BostonHousing2)
s <- summary(lm_BH)$sigma
d <- sapply(m, function(m) pnorm(q, mean = m, sd = s))
nd <- expand.grid(q = q, lp = m)
nd$d <- c(d)

pfun <- function(x, y, z, subscripts, at, ...) {
    panel.contourplot(x, y, z, subscripts, at = 1:9/10, ...)
    panel.xyplot(x = m, y = BostonHousing2$cmedv, pch = 20,   
                 col = rgb(.1, .1, .1, .1),  ...)   
}
p1 <- contourplot(d ~ lp + q, data = nd, panel = pfun, 
                  xlab = "Linear predictor", ylab = "Observed", 
                  main = "Normal Linear Model")

d <- predict(mlt_BH, newdata = BostonHousing2, q = q, type = "distribution")
lp <- c(predict(mlt_BH, newdata = BostonHousing2, q = 1, terms = "bshift"))
nd <- expand.grid(q = q, lp = -lp)
nd$d <- c(d)
pfun <- function(x, y, z, subscripts, at, ...) {
    panel.contourplot(x, y, z, subscripts, at = 1:9/10, ...)
    panel.xyplot(x = -lp, y = BostonHousing2$cmedv, pch = 20, 
                 col = rgb(.1, .1, .1, .1), ...)   
}
p2 <- contourplot(d ~ lp + q, data = nd, panel = pfun, xlab = "Linear predictor", ylab = "Observed", main = "Linear Transformation Model")
grid.arrange(p1, p2, nrow = 1)
@
\caption{Boston Housing. Predicted vs.~observed for the normal linear model (left) and the linear
         transformation model with smooth baseline transformation (right). The observations
         are overlayed with the conditional quantiles of the response given the
         linear predictor. \label{fig:BostonHousing-plot}}
\end{center}
\end{figure}


\paragraph{Truncated Regression: Panel Study of Income Dynamics}

%<<mlt-tobin>>=
%data("tobin", package = "survival")
%ttobin <- subset(tobin, durable > 0)
%(truncreg_tobin <- truncreg(durable ~ age + quant, data = ttobin))
%
%ttobin$durable <- R(ttobin$durable, tleft = 0)
%b_d <- as.basis(~ durable, data = tobin, ui = matrix(c(0, 1), nr  = 1), ci = 0)
%ctm_tobin <- ctm(b_d, shift = ~ age + quant, data = tobin, todistr = "Normal")
%mlt_tobin <- mlt(ctm_tobin, data = ttobin, scale = TRUE)
%logLik(truncreg_tobin)
%logLik(mlt_tobin)	
%-coef(mlt_tobin)[c("(Intercept)", "age", "quant")] / coef(mlt_tobin)["durable"]
%@

\cite{Mroz_1987} analysed the University of Michigan Panel Study of Income
Dynamics (PSID) for the year 1975 (interview year 1976).  The data consists
of 753 married white women between the ages of 30 and 60 in 1975, with 428
working at some time during the year.  The dependent variable is the wife's
annual hours of work and we are interested in modelling the distribution
of hours of work conditional on participation in the labour
force, \ie more than zero hours of work in 1975. We first set-up
a subset consisting of those who actually worked (for money!) in 1975
and a model formula
<<mlt-PSID1976>>=
data("PSID1976", package = "AER")
PSID1976$nwincome <- with(PSID1976, (fincome - hours * wage)/1000)
PSID1976$hours <- as.double(PSID1976$hours)
PSID1976_0 <- subset(PSID1976, participation == "yes")
fm_PSID1976 <- hours ~ nwincome + education + experience + 
                       I(experience^2) + age + youngkids + oldkids
@
We use a 
linear regression model for left-truncated data and compare the
results to \cmd{truncreg} from package \pkg{truncreg} \citep{pkg:truncreg}
<<mlt-PSID1976-truncreg>>=
tr_PSID1976 <- truncreg(fm_PSID1976, data = PSID1976_0)
@

We use the \cmd{R} function (see Appendix~\ref{subapp:R}) to
specify left-truncation at zero, set-up a linear transformation
function with positive slope for the response (we want to stay within 
the normal family) and a shift term
<<mlt-PSID1976-mlt>>=
PSID1976_0$hours <- R(PSID1976_0$hours, tleft = 0)
b_hours <- as.basis(~ hours, data = PSID1976, 
                    ui = matrix(c(0, 1), nr  = 1), ci = 0)
ctm_PSID1976_1 <- ctm(b_hours, shift = fm_PSID1976[-2L], 
                      data = PSID1976_0, todistr = "Normal") 
mlt_PSID1976_1 <- mlt(ctm_PSID1976_1, data = PSID1976_0, scale = TRUE)
@
The \cmd{mlt} function does a slightly better job at maximising the likelihood 
than \cmd{truncreg} which explains the differences in the estimated coefficients
<<mlt-PSID1976-cmpr>>=
logLik(tr_PSID1976)
logLik(mlt_PSID1976_1)
cf <- coef(mlt_PSID1976_1)
RC(truncreg = coef(tr_PSID1976),
   mlt = c(-cf[-grep("hours", names(cf))], 1) / cf["hours"])
@
Of course, we might want to question the normal assumption by allowing a potentially
non-linear transformation function. We simply change the linear to a non-linear
baseline transformation $\h_\rY$ at the price of five additional parameters in the model
<<mlt-PSID1976-mlt-ctm>>=
var_h <- numeric_var("hours", support = range(PSID1976_0$hours$exact),
                     bounds = c(0, Inf))
B_hours <- Bernstein_basis(var_h, order = 6, ui = "increasing")
ctm_PSID1976_2 <- ctm(B_hours, shift = fm_PSID1976[-2L], 
                      data = PSID1976_0, todistr = "Normal")
mlt_PSID1976_2 <- mlt(ctm_PSID1976_2, data = PSID1976_0, 
                      scale = TRUE)
logLik(mlt_PSID1976_2)
@
and there might be a small advantage with the non-normal model.

\subsection{Stratified Linear Transformation Models}

Stratification in linear transformation models refers to a 
strata-specific transformation function but a shift term those
regression coefficients are constant across strata. The model then
reads
\begin{eqnarray*}
\Prob(\rY \le \ry \mid \text{stratum} = s, \rX = \rx) = \pZ(\h(\ry \mid s, \rx)) =
\pZ(\h_\rY(\ry \mid s) - \tilde{\rx}^\top \shiftparm) = \pZ(\basisyx(\ry, s, \rx)^\top \parm)
\end{eqnarray*}
with basis function $\basisyx = (\basisy^\top \otimes
\basisx_\text{stratum}^\top, -\basisx_\text{shift}^\top)^\top$.  The basis
function $\basisx_\text{stratum}^\top$ is a dummy coding for the stratum
variable and is defined using the \code{interacting} argument of \cmd{ctm}. 
The constraints for the parameters of $\basisy$ have to be met for each
single stratum, \ie the total number of linear constraints is the number of
constraints for $\basisy$ multiplied by the number of strata.

\subsubsection{Discrete Responses}

\paragraph{Categorical Data Analysis: Chinese Survey (Cont'd)} We first
estimate the distribution of happiness given health without taking any other
explanatory variables into account, \ie by treating health as a stratum variable 
(with dummy coding) but
without any regression coefficients $\shiftparm$ in the model 
<<mlt-CHFLS-3, cache = TRUE>>=
b_health <- as.basis(~ R_health - 1, data = CHFLS)
ctm_CHFLS_3 <- ctm(b_happy, interacting = b_health, todist = "Logistic")
mlt_CHFLS_3 <- mlt(ctm_CHFLS_3, data = CHFLS, scale = TRUE)
logLik(mlt_CHFLS_3)
predict(mlt_CHFLS_3, newdata = mkgrid(mlt_CHFLS_3), type = "distribution")
@
The conditional distribution for happiness given each health category is returned
by \cmd{predict} as a matrix. There is a clear tendency of people being happier with better health. 
We now `adjust' for age and income by including a linear shift term which is
constant across strata
<<mlt-CHFLS-4, cache = TRUE>>=
ctm_CHFLS_4 <- ctm(b_happy, interacting = b_health, shifting = b_R, 
                   todist = "Logistic")
mlt_CHFLS_4 <- mlt(ctm_CHFLS_4, data = CHFLS, scale = TRUE)
coef(mlt_CHFLS_4)[c("R_age", "R_income")]
@
Because the shift basis \code{b\_R} is negative, the effects of both age and income 
on the happiness distribution are towards larger values of happiness, \ie older and richer
people are happier for all health levels (this is, of course, due to the restrictive model
not allowing interactions between health and the other two variables). The ``health-adjusted''
log-odds ratios are now $\Sexpr{round(exp(coef(mlt_CHFLS_4)["R_age"]), 4)}$ for each year of age and
$\Sexpr{round(exp(coef(mlt_CHFLS_4)["R_income"]), 4)}$ for each additional Yuan earned and, 
conditional on health, people are getting happier as they get older \emph{and} richer.

\subsubsection{Continuous Responses}

\paragraph{Survival Analysis: GBSG-2 Trial (Cont'd)}

The Cox model presented in Section~\ref{subsec:ltm} features one baseline
function for all observations, an assumption which we are now going to relax. As
a first simple example, we want to estimate two separate survivor functions 
for the two treatment regimes in the model
\begin{eqnarray*}
(\pMEV, (\bern{10}(\ry)^\top \otimes
       (\I(\text{hormonal therapy}), 1 - \I(\text{hormonal therapy})))^\top, (\parm_1^\top, \parm_2^\top)^\top)
\end{eqnarray*}
Here, the transformation functions $\bern{10}(\ry)^\top \parm_1$ and 
$\bern{10}(\ry)^\top \parm_2$ correspond to the untreated and treated groups,
respectively
<<mlt-GBSG2-4>>=
b_horTh <- as.basis(GBSG2$horTh)
ctm_GBSG2_4 <- ctm(B_GBSG2y, interacting = b_horTh, 
                   todistr = "MinExtrVal")
mlt_GBSG2_4 <- mlt(ctm_GBSG2_4, data = GBSG2)
@
The two survivor functions, along with the corresponding Kaplan-Meier estimates,
are shown in Figure~\ref{GBSG2-strata-plot}, the low-dimensional Bernstein polynomials
produce a nicely smoothed version of the Kaplan-Meier step-functions.

\begin{figure}
\begin{center}
<<mlt-GBSG2-strata-plot, echo = FALSE, results = "hide">>=
nd <- expand.grid(s <- mkgrid(mlt_GBSG2_4, 100))
nd$mlt_S <- c(predict(mlt_GBSG2_4, newdata = s, type = "survivor"))
nd$KM_S <- unlist(predict(prodlim(Surv(time, cens) ~ horTh, data = GBSG2), 
     	             newdata = data.frame(horTh = s$horTh), times = s$y))
plot(nd$y, nd$mlt_S, ylim = c(0, 1), xlab = "Survival time (days)",
     ylab = "Probability", type = "n", las = 1)
with(subset(nd, horTh == "no"), lines(y, mlt_S, col = "grey", lty = 2))
with(subset(nd, horTh == "yes"), lines(y, mlt_S, lty = 2))
with(subset(nd, horTh == "no"), lines(y, KM_S, type = "s", col = "grey"))
with(subset(nd, horTh == "yes"), lines(y, KM_S, type = "s"))
legend(250, 0.4, lty = c(1, 1, 2, 2), col = c("black", "grey", "black", "grey"),
       legend = c("hormonal therapy, KM", "no hormonal therapy, KM", 
                  "hormonal therapy, MLT", "no hormonal therapy, MLT"), bty = "n", cex = .75)
@
\caption{GBSG-2 Trial. Estimated survivor functions 
         by the most likely transformation model (MLT) and the Kaplan-Meier (KM) estimator in the two 
         treatment groups. The plot reproduces
         Figure~4 (left panel) in \cite{Hothorn_Moest_Buehlmann_2016}. \label{GBSG2-strata-plot}}
\end{center}
\end{figure}

In a second step, we allow treatment-specific baseline hazard functions while
estimating a constant age effect in the model 
\begin{eqnarray*}
(\pMEV, (\bern{10}(\ry)^\top \otimes
       (\I(\text{hormonal therapy}), 1 - \I(\text{hormonal therapy})),
       \text{age})^\top, (\parm_1^\top, \parm_2^\top, \beta)).
\end{eqnarray*}

This model is fitted by
<<mlt-GBSG2-5>>=
ctm_GBSG2_5 <- ctm(B_GBSG2y, interacting = b_horTh, shifting = ~ age, 
                   data = GBSG2, todistr = "MinExtrVal")
mlt_GBSG2_5 <- mlt(ctm_GBSG2_5, data = GBSG2, scale = TRUE)
@
The corresponding stratified Cox model with parameter estimation based
on the partial likelihood is
<<mlt-GBSG2-5-coxph>>=
coxph_GBSG2_5 <- coxph(Surv(time, cens) ~ age + strata(horTh), 
                       data = GBSG2)
cf <- coef(coxph_GBSG2_5)
RC(coxph = cf, mlt = coef(mlt_GBSG2_5)[names(cf)])
@
The Cox model fitted via the stratified partial likelihood (\cmd{coxph}) and
the transformation model agree on a positive age effect $\hat{\beta}$, 
\ie older patients seem to survive longer (note that \cmd{coxph} estimates 
a positive shift effect as does the transformation model specified above).

\subsection{Conditional Transformation Models}

The most complex class of models currently supported by the \cmd{ctm} function
allows basis functions of the form
\begin{eqnarray*}
\basisyx = (\basisy^\top \otimes (\basisx_1^\top,\dots, \basisx_J^\top), -\basisx_\text{shift}^\top).
\end{eqnarray*}
The model may include response-varying coefficients (as defined by the basis $\basisy$) corresponding
to the bases $(\basisx_1^\top,\dots, \basisx_J^\top)$ and constant shift effects ($\basisx_\text{shift}$). Such a model
is set-up using \cmd{ctm} with arguments \code{response} (basis $\basisy$), \code{interacting} (basis $(\basisx_1^\top,\dots, \basisx_J^\top)$)
and \code{shifting} (basis $-\basisx_\text{shift}^\top$). It would be conceptually possible
to fit even more complex conditional transformation models of the form
\begin{eqnarray*}
\basisyx = (\basisy_1^\top \otimes \basisx_1^\top, \dots, \basisy_J^\top \otimes \basisx_J^\top)
\end{eqnarray*}
with a less restrictive user interface.


%\begin{defn}[Transformation model]
%The triple $(\pZ, \basisy, \parm)$ is called transformation model.
%\end{defn}

\subsubsection{Discrete Responses}

\paragraph{Categorical Data Analysis: Chinese Survey (Cont'd)} 
In a series of non-proportional odds models for the conditional distribution of happiness
we study the impact of health, age and income on happiness. Similar to the stratified
model \code{ctm_CHFLS_3}, we estimate the conditional distribution of happiness separately
for each health level, but instead of using a dummy coding we use treatment contrasts
<<mlt-CHFLS-5, cache = TRUE>>=
contrasts(CHFLS$R_health) <- "contr.treatment"
b_health <- as.basis(~ R_health, data = CHFLS)
ctm_CHFLS_5 <- ctm(b_happy, interacting = b_health, todist = "Logistic")
mlt_CHFLS_5 <- mlt(ctm_CHFLS_5, data = CHFLS, scale = TRUE)
predict(mlt_CHFLS_5, newdata = mkgrid(mlt_CHFLS_5), type = "distribution")
logLik(mlt_CHFLS_5)
@
The log-likelihood and the fitted distribution are, of course, equivalent but the parameters
allow a direct interpretation of the effect of health relative to the baseline 
category \code{Poor}. In a second step, we fit a non-proportional odds model where the 
effects of age and income are allowed to vary with happiness
<<mlt-CHFLS-6, cache = TRUE>>=
b_R <- as.basis(~ R_age + R_income, data = CHFLS, remove_intercept = TRUE, 
                scale = TRUE)
ctm_CHFLS_6 <- ctm(b_happy, interacting = b_R, todist = "Logistic")  
mlt_CHFLS_6 <- mlt(ctm_CHFLS_6, data = CHFLS, scale = TRUE)
logLik(mlt_CHFLS_6)
@
and finally we include all three variables (health, age and income) allowing 
happiness-varying effects as
<<mlt-CHFLS-7, cache = TRUE>>=
ctm_CHFLS_7 <- ctm(b_happy, interacting = c(h = b_health, R = b_R), 
    todist = "Logistic")  
mlt_CHFLS_7 <- mlt(ctm_CHFLS_7, data = CHFLS, scale = TRUE)
logLik(mlt_CHFLS_7)
@
%Overall, we get
%<<mlt-CHFLS-AIC>>=
%c("1" = AIC(mlt_CHFLS_1), "2" = AIC(mlt_CHFLS_2), "3" = AIC(mlt_CHFLS_3),
%  "4" = AIC(mlt_CHFLS_4), "5" = AIC(mlt_CHFLS_5), "6" = AIC(mlt_CHFLS_6),
%  "7" = AIC(mlt_CHFLS_7))
%@
%and it seems the proportional-odds model with health stratum (\code{mlt_CHFLS_4}) performs best.

\paragraph{Categorical Data Analysis: Iris Data}

For an unordered response in a multi-class problem, the conditional distribution 
can be estimated using a multinomial regression. In a non-proportional odds
model allowing response-specific regression coefficients, the ordering of the response
levels only affects the corresponding regression coefficients but the fitted density
is invariant with respect to the ordering applied as a comparison with
\cmd{multinom} from package \pkg{nnet} \citep{Venables_Ripley_2002,pkg:nnet} for the iris data shows
<<mlt-iris-1>>=
fm_iris <- Species ~ Sepal.Length + Sepal.Width + 
                     Petal.Length + Petal.Width
multinom_iris <- nnet::multinom(fm_iris, data = iris, trace = FALSE)
logLik(multinom_iris)
iris$oSpecies <- ordered(iris$Species)
b_Species <- as.basis(iris$oSpecies)
b_x <- as.basis(fm_iris[-2L], data = iris, scale = TRUE)
ctm_iris <- ctm(b_Species, interacting = b_x,
                todistr = "Logistic")
mlt_iris <- mlt(ctm_iris, data = iris, scale = TRUE)
logLik(mlt_iris)
p1 <- predict(mlt_iris, newdata = iris, q = sort(unique(iris$oSpecies)), 
              type = "density")
p2 <- predict(multinom_iris, newdata = iris, type = "prob")
max(abs(t(p1) - p2))
@
From this point of view, the multinomial model for an unordered categorical
response is equivalent to a non-proportional odds model with
response-varying effects.

\subsubsection{Continuous Responses}

\paragraph{Survival Analysis: GBSG-2 Trial (Cont'd)}

The Cox model for the comparison of the survivor distribution between the
untreated and treated group assuming proportional hazards, \ie the model
$(\pMEV, (\bern{10}^\top, \I(\text{hormonal therapy}))^\top, (\parm_1^\top, \beta)^\top)$,
implements the transformation function $\h(\ry \mid \text{treatment}) =
\bern{10}(\ry)^\top \parm_1 + \I(\text{hormonal therapy}) \beta$ where
$\bern{10}^\top \parm_1$ is the log-cumulative baseline hazard function
parameterised by a Bernstein polynomial and $\beta \in \RR$ is the
log-hazard ratio of hormonal therapy
<<mlt-GBSG2-6>>=
ctm_GBSG2_6 <- ctm(B_GBSG2y, shifting = ~ horTh, data = GBSG2, 
                   todistr = "MinExtrVal")
mlt_GBSG2_6 <- mlt(ctm_GBSG2_6, data = GBSG2)
logLik(mlt_GBSG2_6)
@

This is the classical Cox model with one treatment parameter $\beta$ 
but fully parameterised baseline transformation function which was fitted by the exact
log-likelihood under ten linear constraints. The model assumes proportional hazards, 
an assumption whose
appropriateness we want to assess using the non-proportional hazards model
$(\pMEV, (\bern{10}^\top \otimes (1, \I(\text{hormonal therapy})))^{\top}, \parm)$ with
transformation function 
\begin{eqnarray*}
\h(\ry \mid \text{treatment}) = \bern{10}(\ry)^\top \parm_1 + \I(\text{hormonal therapy}) \bern{10}(\ry)^\top \parm_2. 
\end{eqnarray*}
The function $\bern{10}^\top \parm_2$ is the time-varying treatment effect
and can be interpreted as the deviation, on the scale of the transformation
function, induced by the hormonal therapy.  Under the null hypothesis of no
treatment effect, we would expect $\parm_2 \equiv \bold{0}$.  It should be
noted that the log-cumulative hazard function $\bern{10}(\ry)^\top \parm_1$
must by monotone. The deviation function $\bern{10}(\ry)^\top \parm_2$ 
does not need to be monotone, however, the sum of the two functions needs to
be monotone. Sums of such Bernstein polynomials with coefficients $\parm_1$ and
$\parm_2$ are again Bernstein polynomials with coefficients $\parm_1 + \parm_2$.
Thus, monotonicity of the sum can be implemented by monotonicity of $\parm_1 +
\parm_2$. The argument \code{sumconstr = TRUE} implements the latter
constraint (the default, in this specific situation).
Figure~\ref{GBSG2-deviation-plot} shows the time-varying treatment effect
$\bern{10}^\top \hat{\parm}_{2, N}$, together with a $95\%$ confidence band
(see Section~\ref{sec:asympt} for a description of the method). The $95\%$
confidence interval around the log-hazard ratio $\hat{\beta}$ is plotted in
addition and since the latter is fully covered by the confidence band for
the time-varying treatment effect there is no reason to question the
treatment effect computed under the proportional hazards assumption.

<<mlt-GBSG2-7>>=
b_horTh <- as.basis(~ horTh, data = GBSG2)
ctm_GBSG2_7 <- ctm(B_GBSG2y, interacting = b_horTh, 
                   todistr = "MinExtrVal")
nd <- data.frame(y = GBSG2$time[1:2], horTh = unique(GBSG2$horTh))
attr(model.matrix(ctm_GBSG2_7, data = nd), "constraint")
mlt_GBSG2_7 <- mlt(ctm_GBSG2_7, data = GBSG2)
logLik(mlt_GBSG2_7)
@

\begin{figure}[t!]
\begin{center} 
<<mlt-GBSG2-deviation-plot, echo = FALSE, results = "hide">>=

s <- mkgrid(mlt_GBSG2_7, 15)
s$y <- s$y[s$y > 100 & s$y < 2400]
nd <- expand.grid(s)
K <- model.matrix(ctm_GBSG2_7, data = nd)
Kyes <- K[nd$horTh == "yes",]
Kyes[,grep("Intercept", colnames(K))] <- 0  
gh <- glht(parm(coef(mlt_GBSG2_7), vcov(mlt_GBSG2_7)), Kyes)
ci <- confint(gh)
coxy <- s$y

K <- matrix(0, nrow = 1, ncol = length(coef(mlt_GBSG2_6)))
K[,length(coef(mlt_GBSG2_6))] <- 1
ci2 <- confint(glht(mlt_GBSG2_6, K))

plot(coxy, ci$confint[, "Estimate"], ylim = range(ci$confint), type = "n",
     xlab = "Survival time (days)", ylab = "Log-hazard ratio", las = 1)
polygon(c(coxy, rev(coxy)), c(ci$confint[,"lwr"], rev(ci$confint[, "upr"])),
        border = NA, col = rgb(.1, .1, .1, .1))
lines(coxy, ci$confint[, "Estimate"], lty = 1, lwd = 1)
lines(coxy, rep(ci2$confint[,"Estimate"], length(coxy)), lty = 2, lwd = 1) 
lines(coxy, rep(0, length(coxy)), lty = 3)
polygon(c(coxy[c(1, length(coxy))], rev(coxy[c(1, length(coxy))])),
        rep(ci2$confint[,c("lwr", "upr")], c(2, 2)),
        border = NA, col = rgb(.1, .1, .1, .1))
legend("bottomright", lty = 1:2, lwd = 1, legend = c("time-varying log-hazard ratio",
       "time-constant log-hazard ratio"), bty = "n", cex = .75)
@
\caption{GBSG-2 Trial. Verification of proportional hazards: The log-hazard ratio $\hat{\beta}$
         (dashed line) with $95\%$ confidence interval (dark grey) is fully 
         covered by a $95\%$ confidence band for the time-varying treatment effect (light grey,
         the estimate is the solid line)
         computed from a non-proportional hazards model. 
         The plot reproduces
         Figure~4 (right panel) in \cite{Hothorn_Moest_Buehlmann_2016}. \label{GBSG2-deviation-plot}}
\end{center}
\end{figure}   

In a second step, we allow an age-varying treatment effect 
in the model $(\pMEV, (\bern{10}(\ry)^\top \otimes
       (\I(\text{hormonal therapy}), 1 - \I(\text{hormonal therapy}))
       \otimes \bernx{3}(\text{age})^\top)^\top, \parm)$. For both treatment
groups, we estimate a conditional transformation function of survival time
$\ry$ given age parameterised as the tensor basis of two Bernstein bases. Each of the
two basis functions comes with $10 \times 3$ linear constraints, so the model
was fitted under $60$ linear constraints
%% <FIXME> use quantile(.1, .9) instead of range for age??? </FIXME>
<<mlt-GBSG2-8, echo = TRUE, cache = TRUE>>=
var_a <- numeric_var("age", support = range(GBSG2$age))
B_age <- Bernstein_basis(var_a, order = 3)
b_horTh <- as.basis(GBSG2$horTh)
ctm_GBSG2_8 <- ctm(B_GBSG2y, 
                   interacting = b(horTh = b_horTh, age = B_age), 
                   todistr = "MinExtrVal")
mlt_GBSG2_8  <- mlt(ctm_GBSG2_8, data = GBSG2)
logLik(mlt_GBSG2_8)
@
Figure~\ref{fig:GBSG2-8-plot} allows an assessment of the prognostic and
predictive properties of age.  As the survivor functions are clearly larger
under hormonal treatment for all patients, the positive treatment effect
applies to all patients.  However, the size of the treatment effect varies
greatly.  For women younger than $30$, the effect is most pronounced and
levels-off a little for older patients.  In general, the survival times are
longest for women between $40$ and $60$ years old.  Younger women suffer
the highest risk; for women older than $60$ years, the risk starts to
increase again.  This effect is shifted towards younger women by the
application of hormonal treatment.

\begin{figure}
\begin{center}
<<mlt-GBSG2-8-plot, echo = FALSE>>=
nlev <- c(no = "without hormonal therapy", yes = "with hormonal therapy")
levels(nd$horTh) <- nlev[match(levels(nd$horTh), names(nlev))]
s <- mkgrid(mlt_GBSG2_8, 100)
nd <- expand.grid(s)
nd$s <- c(predict(mlt_GBSG2_8, newdata = s, type = "survivor"))
contourplot(s ~ age + y | horTh, data = nd, at = 1:9 / 10,
            ylab = "Survival time (days)", xlab = "Age (years)",
            scales = list(x = list(alternating = c(1, 1))))
@
\caption{GBSG-2 Trial. Prognostic and predictive effect of age. The contours depict the
         conditional survivor functions given treatment and age of the patient. 
         The plot reproduces
         Figure~5 in \cite{Hothorn_Moest_Buehlmann_2016}.
         \label{fig:GBSG2-8-plot}}
\end{center}
\end{figure}

\paragraph{Quantile Regression: Head Circumference}

The Fourth Dutch Growth Study \citep{Fredriks_Buuren_Burgmeijer_2000} is a
cross-sectional study on growth and development of the Dutch population
younger than $22$ years.  \cite{Stasinopoulos_Rigby_2007} fitted a growth
curve to head circumferences (HC) of $7040$ boys using a GAMLSS model with a
Box-Cox $t$ distribution describing the first four moments of head
circumference conditionally on age.  The model showed evidence of kurtosis,
especially for older boys.  We fit the same growth curves by the
conditional transformation model $(\Phi, (\bern{3}(\text{HC})^\top \otimes
\bernx{3}(\text{age}^{1/3})^\top)^\top, \parm)$ by maximisation of the
approximate log-likelihood under $3 \times 4$ linear constraints
<<mlt-head, echo = TRUE, cache = TRUE>>=
data("db", package = "gamlss.data")
db$lage <- with(db, age^(1/3))
var_head <- numeric_var("head", support = quantile(db$head, c(.1, .9)),
                       bounds = range(db$head))
B_head <- Bernstein_basis(var_head, order = 3, ui = "increasing")
var_lage <- numeric_var("lage", support = quantile(db$lage, c(.1, .9)),
                        bounds = range(db$lage))
B_age <- Bernstein_basis(var_lage, order = 3, ui = "none")
ctm_head <- ctm(B_head, interacting = B_age)
mlt_head <- mlt(ctm_head, data = db, scale = TRUE)
@
Figure~\ref{fig:head-plot} shows the
data overlaid with quantile curves obtained via inversion of the estimated
conditional distributions.  The figure very closely reproduces the growth
curves presented in Figure~16 of \cite{Stasinopoulos_Rigby_2007} and also
indicates a certain asymmetry towards older boys.

\begin{figure}
\begin{center}
<<mlt-head-plot, echo = FALSE>>=
pr <- expand.grid(s <- mkgrid(ctm_head, 100))
pr$p <- c(predict(mlt_head, newdata = s, type = "distribution"))
pr$lage <- pr$lage^3
pr$cut <- factor(pr$lage > 2.5)
levels(pr$cut) <- c("Age < 2.5 yrs", "Age > 2.5 yrs")
pfun <- function(x, y, z, subscripts, at, ...) {
    panel.contourplot(x, y, z, subscripts,
        at = c(0.4, 2, 10, 25, 50, 75, 90, 98, 99.6)/ 100, ...)
    panel.xyplot(x = db$age, y = db$head, pch = 20,
                 col = rgb(.1, .1, .1, .1), ...)
}
print(contourplot(p ~ lage + head | cut, data = pr, panel = pfun, region = FALSE,
            xlab = "Age (years)", ylab = "Head circumference (cm)",
            scales = list(x = list(relation = "free"))))
@
\caption{Head Circumference Growth. Observed head circumference and age for
         $7040$ boys with estimated quantile curves for
         $\tau = 0.04, 0.02, 0.1, 0.25, 0.5, 0.75, 0.9, 0.98, 0.996$.
         The plot reproduces
         Figure~3 in \cite{Hothorn_Moest_Buehlmann_2016}.
         \label{fig:head-plot}}
\end{center}
\end{figure}

\paragraph{Non-normal Linear Regression: Boston Housing Data (Cont'd)}

A response-varying coefficient model, also called distribution regression \citep{Foresi_Peracchi_1995,
Chernozhukov_2013,Koenker_Leorato_Peracchi_2013}, for the Boston Housing data is 
\begin{eqnarray*}
\Prob(\text{medv} \le \ry \mid \rX = \rx) & = &  
  \Phi\left(\h_\rY(\ry) - \sum_{j = 1}^J \shiftparm_j(\ry) \tilde{\rx}_j\right) \\
& = & \Phi\left(\bern{6}(\ry)^\top \parm_0 - \sum_{j = 1}^J \bern{6}(\ry)^\top \parm_j \tilde{\rx}_j\right) \\
& = & \Phi\left(\bern{6}(\ry)^\top \left(\parm_0 - \sum_{j = 1}^J \parm_j \tilde{\rx}_j\right)\right) \\
& = & \Phi\left(\bern{6}(\ry)^\top \left(\parm_0 - \parm(\rx)\right)\right)
\end{eqnarray*}
The model requires the parameters $\parm_0 - \parm(\rx)$ to be monotone
increasing for all possible values of $\rx$. This type of constraint can be
implemented using the \code{sumconstr = TRUE} argument to \cmd{ctm}.
The model is implemented using the basis function 
$\basisyx = (\bern{6}^\top \otimes (1, \tilde{\rx}^\top))^\top$, the
intercept is required here and $\tilde{\rx}$ should be scaled to the unit
interval. This model, here using parameters $\parm_0 + \parm(\rx)$,
can be fitted using 
<<mlt-BostonHousing-dr-sumconstr, cache = TRUE>>=
b_BH_s <- as.basis(fm_BH[-2L], data = BostonHousing2, scale = TRUE)
ctm_BHi <- ctm(B_m, interacting = b_BH_s, sumconstr = TRUE)
mlt_BHi <- mlt(ctm_BHi, data = BostonHousing2)
logLik(mlt_BHi)
@
This takes quite some time, simply because the number of constraints is
is very large, depending on the number of explanatory variables. It might
make sense to restrict all partial functions, and thus all partial
parameters $\parm_j$ to be monotone (argument \code{sumconstr = FALSE}):
<<mlt-BostonHousing-dr, cache = TRUE>>=
ctm_BHi2 <- ctm(B_m, interacting = b_BH_s, sumconstr = FALSE)
mlt_BHi2 <- mlt(ctm_BHi2, data = BostonHousing2)
logLik(mlt_BHi2)
@

Figure~\ref{fig:Boston-Housing-dr-plot} compares the fitted densities for
the linear transformation model with constant regression coefficients
\code{mlt_BH} and the two distribution regression models \code{mlt_BHi} and
\code{mlt_BHi2}.  For some observations, the variance of the conditional
distribution functions seems to be smaller in the more complex model
distribution regression models with response-varying effects, compared to a
linear transformation model with constant shift effects $\shiftparm$. There
is not very much difference between the distribution regression models with
different constraints.

\begin{figure}
\begin{center}
<<mlt-Boston-Housing-dr-plot, echo = FALSE, fig.height = 3>>=
q <- mkgrid(var_m, 100)[[1]]
tr <- predict(mlt_BH, newdata = BostonHousing2[, all.vars(fm_BH[-2L])],
              q = q, type = "density")
tri <- predict(mlt_BHi, newdata = BostonHousing2[, all.vars(fm_BH[-2L])],
              q = q, type = "density")
tri2 <- predict(mlt_BHi2, newdata = BostonHousing2[, all.vars(fm_BH[-2L])],
              q = q, type = "density")
layout(matrix(1:3, ncol = 3))
Q <- matrix(q, nrow = length(q), ncol = ncol(tr))
ylim <- range(c(tr, tri))
matplot(Q, tr, ylim = ylim, xlab = "Median Value", ylab = "Density",
        type = "l", col = rgb(.1, .1, .1, .1), lty = 1, main = "mlt_BH") 
matplot(Q, tri, ylim = ylim, xlab = "Median Value", ylab = "Density",
        type = "l", col = rgb(.1, .1, .1, .1), lty = 1, main = "mlt_BHi")
matplot(Q, tri2, ylim = ylim, xlab = "Median Value", ylab = "Density",
        type = "l", col = rgb(.1, .1, .1, .1), lty = 1, main = "mlt_BHi2")
@
\caption{Boston Housing. Fitted conditional densities for the linear transformation model
         with constant regression coefficients (\code{mlt\_BH}) and the response-varying
         coefficient models \code{mlt\_BHi} and \code{mlt\_BHi2}, the last model 
         assumes monotone response-varying coefficient functions.
         \label{fig:Boston-Housing-dr-plot}}
\end{center}
\end{figure}


\subsubsection{Count Responses}

Finally, we study a transformation model with response-varying coefficients
for count data.

\paragraph{Analysis of Count Data: Tree Pipit Counts}

\cite{Mueller_Hothorn_2004} reported data on the number of tree pipits
\textit{Anthus trivialis}, a small passerine bird, counted on $86$ forest plots at a light gradient
ranging from open and sunny stands (small cover storey) to dense and dark
stands (large cover storey).  We model the conditional distribution of the
number of tree pipits at one plot given the cover storey at this plot 
by the transformation
model $(\Phi, (\basisy^\top \otimes \bernx{4}(\text{cover storey})^\top)^\top,
\parm)$, where $\basisy(y) = \evec_5(y + 1), y = 0, \dots, 4$; the model
is fitted under $4 \times 5$ linear constraints. In this
model for count data, the conditional distribution depends on
both the number of counted birds and the cover storey and the effect of
cover storey may change with different numbers of birds observed
%% <FIXME> var ordered </FIXME>
%% <FIXEM> force spg as auglag doesn't seem to work here </FIXME>
<<mlt-treepipit, echo = TRUE, cache = TRUE>>=
data("treepipit", package = "coin")
treepipit$ocounts <- ordered(treepipit$counts)
B_cs <- Bernstein_basis(var = numeric_var("coverstorey", support = 1:110), 
                        order = 4)
B_c <- as.basis(treepipit$ocounts)
ctm_treepipit <- ctm(B_c, interacting = B_cs)
mlt_treepipit <- mlt(ctm_treepipit, data = treepipit, scale = TRUE,
                     optim = mltoptim()["spg"])
@
The left panel of Figure~\ref{fig:treepipit-plot} depicts the observations and the
center panel shows the conditional distribution function evaluated for $0,
\dots, 5$ observed birds.  The conditional distribution function obtained
from a generalised additive Poisson (GAM) model with smooth mean effect of cover
storey \citep[computed using \pkg{mgcv},][]{Wood_2006,pkg:mgcv} is given in the right panel  
<<mlt-mgcv, echo = FALSE, results = "hide">>=
library("mgcv") ### masks nnet::multinom
@
<<mlt-treepipit-gam>>=
gam_treepipit <- gam(counts ~ s(coverstorey), data = treepipit, 
                     family = "poisson")
@
Despite some overfitting, this model is
more restrictive than the transformation model because one mean function determines the whole distribution
(the local minima of the conditional distributions as a function of cover storey are
constant in the right panel whereas they are shifted towards higher values of
cover storey in the center panel).

\begin{figure}
\begin{center}
<<mlt-treepipit-plot, echo = FALSE, fig.height = 3>>=
s <- mkgrid(ctm_treepipit, 100)
s$ocounts <- s$ocounts[1:5]
nd <- expand.grid(s)
nd$p <- c(predict(mlt_treepipit, newdata = s, type = "distribution"))

### produce a table
tpt <- xtabs(~ counts + coverstorey, data = treepipit)

### construct a data frame with frequencies
treepipit2 <- sapply(as.data.frame(tpt, stringsAsFactors = FALSE),
                     as.integer)

s <- mkgrid(ctm_treepipit, 10)
s$ocounts <- s$ocounts[1]
K <- model.matrix(ctm_treepipit, data = expand.grid(s))
#g <- glht(parm(coef(mod), vcov(mod)), linfct = K)
#confint(g)
nd$lambda <- predict(gam_treepipit, newdata = nd, type = "response")

layout(matrix(1:3, nr = 1))
par("mai" = par("mai") * c(1, .95, 1, .85))
xlim <- range(treepipit[, "coverstorey"]) * c(0.98, 1.05)
xlab <- "Cover storey"
ylab <- "Number of tree pipits (TP)"
### scatterplot again; plots are proportional to number of plots
plot(counts ~ coverstorey, data = treepipit2, cex = sqrt(Freq),
     ylim = c(-.5, 5), xlab = xlab, ylab = ylab, col = "darkgrey", 
     xlim = xlim, las = 1, main = "Observations")

plot(c(0, 110), c(0, 1), type = "n", xlab = xlab, ylab = "Conditional probability",
     xlim = xlim, las = 1, main = "MLT")
with(subset(nd, ocounts == "0"), lines(coverstorey, p, lty = 1))
with(subset(nd, ocounts == "1"), lines(coverstorey, p, lty = 2))
with(subset(nd, ocounts == "2"), lines(coverstorey, p, lty = 3))
with(subset(nd, ocounts == "3"), lines(coverstorey, p, lty = 4))
with(subset(nd, ocounts == "4"), lines(coverstorey, p, lty = 5))
abline(h = 1, lty = 6)
legend("bottomright", lty = 1:6, legend = c(expression(TP == 0),
                                            expression(TP <= 1),
                                            expression(TP <= 2),
                                            expression(TP <= 3),
                                            expression(TP <= 4),
                                            expression(TP <= 5)), bty = "n")

plot(c(0, 110), c(0, 1), type = "n", xlab = xlab, ylab = "Conditional probability",
     xlim = xlim, las = 1, main = "GAM")
with(subset(nd, ocounts == "0"), lines(coverstorey, ppois(0, lambda), lty = 1))
with(subset(nd, ocounts == "1"), lines(coverstorey, ppois(1, lambda), lty = 2))
with(subset(nd, ocounts == "2"), lines(coverstorey, ppois(2, lambda), lty = 3))
with(subset(nd, ocounts == "3"), lines(coverstorey, ppois(3, lambda), lty = 4))
with(subset(nd, ocounts == "4"), lines(coverstorey, ppois(4, lambda), lty = 5))
abline(h = 1, lty = 6)
@
\caption{Tree Pipit Counts. Observations (left panel, the size of the points is
         proportional to the number of observations) and estimated conditional distribution
         of number of tree pipits given cover storey by the most likely transformation model (MLT, center panel)
         and a generalised additive Poisson model (function \cmd{gam} in package \pkg{mgcv}, 
         GAM, right panel). The plot reproduces
         Figure~7 in \cite{Hothorn_Moest_Buehlmann_2016}. \label{fig:treepipit-plot}}
\end{center}
\end{figure}


\section{Most Likely Transformations} \label{sec:mlt}

In this Section we review the underpinnings of the \cmd{mlt} function 
implementing the \emph{most likely transformation} estimator. Most of the material in this section
originates from \cite{Hothorn_Moest_Buehlmann_2017}.
For a given transformation function $h$, the likelihood contribution of a
datum $\esAY = (\ubar{\ry},\bar{\ry}] \in \sAY$ is defined in terms of the
distribution function \citep{Lindsey_1996}: 
\begin{eqnarray*} 
\lik(\h \mid \rY \in \esAY) := \int_{\esAY} \dY(y \mid \h) d\measureY(y) =
\pZ(\h(\bar{\ry})) - \pZ(\h(\ubar{\ry})).
\end{eqnarray*}
This ``exact'' definition of the likelihood applies to most practically interesting
situations and, in particular, allows discrete and (conceptually) continuous as well as
censored or truncated observations $\esAY$. For a discrete response $\ry_k$ we have $\bar{\ry} = \ry_k$ and $\ubar{\ry}
= \ry_{k -1}$ such that $\lik(\h \mid \rY = \ry_k) = \dY(\ry_k \mid \h) =
\pZ(\h(\bar{\ry})) - \pZ(\h(\ubar{\ry}))$.  For absolutely continuous random
variables $\rY$ we always practically observe an imprecise datum $(\ubar{\ry},\bar{\ry}]
\subset \RR$ and, for short intervals $(\ubar{\ry},\bar{\ry}]$, approximate
the exact likelihood $\lik(\h \mid \rY \in (\ubar{\ry},\bar{\ry}])$ by the term
$(\bar{\ry} - \ubar{\ry}) \dY(\ry \mid \h)$ or simply $\dY(\ry \mid \h)$ with $\ry
= (\ubar{\ry} + \bar{\ry})/2$ \citep{Lindsey_1999}. This approximation only
works for relatively precise measurements, \ie short intervals. If longer intervals 
are observed, one speaks of ``censoring'' and relies on the exact definition
of the likelihood contribution instead of using the above approximation \citep{Klein_Moeschberger_2003}. 
In summary, the likelihood contribution of a conceptually ``exact
continuous'' or left, right or interval-censored continuous or discrete 
observation $(\ubar{\ry}, \bar{\ry}]$ is given by 
\begin{eqnarray*}
\lik(\h \mid \rY \in (\ubar{\ry}, \bar{\ry}]) \left\{ \begin{array}{ll}
    \approx \dZ(\h(\ry)) \h^\prime(\ry) & \ry = (\ubar{\ry} + \bar{\ry})/2 \in \samY \quad \text{```exact continuous'''}\\
    = 1 - \pZ(\h(\ubar{\ry})) & \ry \in (\ubar{\ry}, \infty) \cap \samY \quad \text{`right-censored'} \\
    = \pZ(\h(\bar{\ry})) & \ry \in (-\infty, \bar{\ry}] \cap \samY \quad \text{`left-censored'} \\
    = \pZ(\h(\bar{\ry})) - \pZ(\h(\ubar{\ry})) & \ry \in (\ubar{\ry},\bar{\ry}] \cap \samY \quad    \text{`interval-censored',}
\end{array} \right. 
\end{eqnarray*}
under the assumption of random censoring.
The likelihood is more complex under dependent censoring
\citep{Klein_Moeschberger_2003} but this is is not covered by the \pkg{mlt} implementation. 
The likelihood contribution $\lik(\h \mid \rY \in (\ry_k, \ry_{k-1}])$
of an ordered factor in category $\ry_k$ is equivalent to the term 
$\lik(\h \mid \rY \in (\ubar{\ry}, \bar{\ry}])$
contributed by an interval-censored observation $(\ubar{\ry},\bar{\ry}]$
when category $\ry_k$ was defined by the interval $(\ubar{\ry},\bar{\ry}]$. Thus,
the expression $\pZ(\h(\bar{\ry})) - \pZ(\h(\ubar{\ry}))$ for the likelihood
contribution reflects the equivalence of interval-censoring 
and categorisation at corresponding cut-off points. 

For truncated observations in the interval $(\ry_l, \ry_r] \subset \samY$,
the above likelihood contribution is defined in terms of the 
distribution function conditional on the truncation
\begin{eqnarray*}
\pY(\ry \mid \rY \in (\ry_l, \ry_r]) = \pZ(\h(\ry) \mid \rY \in (\ry_l, \ry_r]) = 
\frac{\pZ(\h(\ry))}{\pZ(\h(\ry_r)) - \pZ(\h(\ry_l))}  
\quad \forall \ry \in (\ry_l, \ry_r]
\end{eqnarray*}
and thus the likelihood contribution changes to \citep{Klein_Moeschberger_2003}
\begin{eqnarray*}
\frac{\lik(\h \mid \rY \in (\ubar{\ry}, \bar{\ry}])}{\pZ(\h(\ry_r)) -
\pZ(\h(\ry_l))} = \frac{\lik(\h \mid \rY \in (\ubar{\ry}, \bar{\ry}])}{\lik(\h
\mid \rY \in (\ry_l, \ry_r])} \quad \text{when } \ry_l < \ubar{\ry} < \bar{\ry} \le \ry_r.
\end{eqnarray*}
It is important to note that the likelihood is always \textit{defined} in
terms of a distribution function \citep{Lindsey_1999} and it therefore makes
sense to directly model the distribution function of interest.  The ability
to uniquely characterise this distribution function by the
transformation function $\h$ gives rise to the following definition of an
estimator $\hat{\h}_N$. For an independent sample of possibly
censored or truncated observations $\esAY_1, \dots, \esAY_N$ 
from $\Prob_\rY$ the estimator
\begin{eqnarray*}
\hat{\h}_N := \argmax_{\tilde{\h} \in \hs} \sum_{i = 1}^{N} \log(\lik(\tilde{\h} \mid
\rY \in \esAY_i)) 
\end{eqnarray*}
is called the most likely transformation (MLT). In \pkg{mlt}, we
parameterise the transformation function $\h(\ry)$ as a linear function of
its basis-transformed argument $\ry$ using a basis function $\basisy: \samY
\rightarrow \RR^\dimparm$ such that $\h(\ry) = \basisy(\ry)^\top \parm,
\parm \in \RR^\dimparm$.  The choice of the basis function $\basisy$ is
problem-specific, practically relevant examples were discussed in Section~\ref{sec:trafo}.  In the conditional
case we use the basis $\basisyx(\ry, \rx)$ instead of $\basisy(\ry)$. The
likelihood $\lik$ only requires evaluation of $\h$, and only an
approximation thereof using the Lebesgue density of ``exact continuous''
observations makes the evaluation of the first derivative of $\h(\ry)$ with
respect to $\ry$ necessary.  In this case, the derivative with respect to
$\ry$ is given by $\h^\prime(\ry) = \basisy^\prime(\ry)^\top \parm$ and we
assume that $\basisy^\prime$ is available.  In the following we write $\h =
\basisy^\top \parm$ and $\h^\prime = {\basisy^\prime}^\top \parm$ for the
transformation function and its first derivative omitting the argument $\ry$
and we assume that both functions are bounded away from $-\infty$ and
$\infty$. For the basis functions discussed in Section~\ref{sec:trafo}, the 
constraint $\parm \in \Theta$ can be written as $\mC \parm \ge \bold{0}$, thus
the solution to the optimisation problem
\begin{eqnarray*}
\hat{\parm}_N := \argmax_{\mC \parm \ge \bold{0}} \sum_{i = 1}^N
\log(\lik(\basisy^\top \parm \mid \rY \in \esAY_i))
\end{eqnarray*}
is the maximum likelihood estimator. The plug-in estimator for the most
likely transformation is $\hat{\h}_N := \basisy^\top \hat{\parm}_N$.  

The score contribution of an ``exact continuous'' 
observation $\ry = (\ubar{\ry} +
\bar{\ry})/2$ from an absolutely continuous distribution is approximated by the
gradient of the log-density
\begin{eqnarray*}
\s(\parm \mid \rY \in (\ubar{\ry}, \bar{\ry}]) \approx
\frac{\partial \log(\dY(\ry \mid \parm))}{\partial \parm} & = & 
\frac{\partial \log(\dZ(\basisy(\ry)^\top \parm))) +
\log({\basisy^\prime(\ry)}^\top \parm)}{\partial \parm} \nonumber \\
& = & \basisy(\ry) \frac{\dZ^\prime(\basisy(\ry)^\top \parm)}{\dZ(\basisy(\ry)^\top \parm)}
    + \frac{\basisy^\prime(\ry)}{{\basisy^\prime(\ry)}^\top \parm}. \label{f:s_exact}
\end{eqnarray*}
For an interval-censored or discrete observation $\ubar{\ry}$ and
$\bar{\ry}$ (the constant terms $\pZ(\basisy(\pm \infty)^\top \parm) =
\pZ(\pm \infty) = 1$ or $0$ vanish) the score contribution is
\begin{eqnarray*}
\s(\parm \mid \rY \in (\ubar{\ry}, \bar{\ry}]) & = & \frac{\partial \log(\lik(\basisy^\top \parm \mid \rY \in (\ubar{\ry},
\bar{\ry}]))}{\partial \parm} \nonumber \\
& = & \frac{\partial \log(\pZ(\basisy(\bar{\ry})^\top \parm) - \pZ(\basisy(\ubar{\ry})^\top \parm))}{\partial \parm}  \nonumber \\
& = & \frac{\dZ(\basisy(\bar{\ry})^\top \parm)\basisy(\bar{\ry}) - \dZ(\basisy(\ubar{\ry})^\top
\parm) \basisy(\ubar{\ry})}{\pZ(\basisy(\bar{\ry})^\top \parm) - \pZ(\basisy(\ubar{\ry})^\top
\parm)}. \label{f:s_interval}
\end{eqnarray*}
For a truncated observation, the score function is $\s(\parm \mid \rY \in
(\ubar{\ry}, \bar{\ry}]) - \s(\parm \mid \rY \in (\ry_l, \ry_r])$.
%\begin{eqnarray*}
%\frac{\partial -\log[\pZ(\basisy(\ry_r)^\top \parm) - \pZ(\basisy(\ry_l)^\top
%\parm)]}{\partial \parm} & = &
%-\frac{\dZ(\basisy(\ry_r)^\top \parm)\basisy(\ry_r) - \dZ(\basisy(\ry_l)^\top \parm)\basisy(\ry_l)}
%     {\pZ(\basisy(\ry_r)^\top \parm) - \pZ(\basisy(\ry_l)^\top \parm)}
%\end{eqnarray*}
%has to be added to the score contribution $\s(\parm \mid \rY \in (\ubar{\ry},
%\bar{\ry}])$.

The \cmd{mlt} function uses the convenience interfaces 
\cmd{auglag} for augmented Lagrangian minimization 
\citep[package \pkg{alabama}]{pkg:alabama} and 
\cmd{BB} for spectral projected gradients implemented in the \code{spg()} function of
package \pkg{BB} \citep{Varadhan_Gilbert_2009, pkg:BB} for maximising this
log-likelihood.  Starting values are obtained from an estimate of the
unconditional distribution function $\Prob(\rY \le \ry)$ (for example, the
empirical cumulative distribution function, a Kaplan-Meier or Turnbull estimate) via a
(constrained) linear regression of $\rz_i = \pZ^{-1}(\Prob(\rY \le \ry_i))$ on the
design matrix of the transformation model. Optionally, columns of the
underlying model matrices are scaled to $[-1, 1]$ (argument \code{scale = TRUE}
to \cmd{mlt}) which leads to considerable faster optimisation in many cases.
% Additional arguments (\code{...}) to \cmd{mlt}, such as the maximum number of iterations 
% \code{maxit}, are forwarded to \cmd{spg}.

\section{Transformation Analysis} \label{sec:predict}

Based on the maximum likelihood estimator $\hat{\parm}_N$ and the most
likely transformation is $\hat{\h}_N$, transformation models can be analysed on different scales.
Plug-in estimators for the distribution and cumulative hazard functions are given by $\hatpY = \pZ
\circ \basisy^\top \hat{\parm}_N$ and $\hatHazY = -\log(1 - \hatpY)$.  For a
continuous model, the density is given by $\hatdY = \dZ \circ \basisy^\top
\hat{\parm}_N \times {\basisy^\prime}^\top \hat{\parm}_N$ and the 
quantile function is obtained by numerical inversion of the distribution
function. For a discrete model
we get $\hatdY(\ry_k) = \pZ(\basisy(\ry_k)^\top \hat{\parm}_N) - 
\pZ(\basisy(\ry_{k - 1})^\top \hat{\parm}_N)$ (with $\pZ(\basisy(\ry_{0})^\top \hat{\parm}_N) := 0$ and
$\pZ(\basisy(\ry_{K})^\top \hat{\parm}_N) := 1$). The hazard function is 
$\hathazY = \hatdY / (1 - \hatpY)$.

The \cmd{predict} method for \code{mlt} objects is the main user interface 
for the evaluation of these functions, the type of which is selected
by its \code{type} argument. Conceptually, all functions are evaluated
on the support of $\rY$, \ie on a grid $\ry_1, \dots, \ry_K$ 
(arguments \code{q} for the vector of grid points or \code{K} for the number
of grid point to be generated) for continuous 
responses for observations with explanatory variables as given in the
\code{newdata} argument. That means the transformation function
\begin{eqnarray*}
\hat{\h}_N(\ry_k \mid \rx_i) = \basisyx(\ry_k, \rx_i)^\top \hat{\parm}_N, 
\quad k = 1, \dots, K; i = 1, \dots, N_\text{new}
\end{eqnarray*}
is evaluated for potentially large numbers $K$ and $N_\text{new}$ by \cmd{predict}
and is returned as a $K \times N_\text{new}$ matrix (columns correspond to
observations). Because in the
most general case of a conditional distribution function the transformation
function is 
\begin{eqnarray*}
\hat{\h}_N(\ry_k \mid \rx_i) = 
(\basisy_1(\ry_k)^\top \otimes (\basisx_1(\rx_i)^\top,\dots, \basisx(\rx_i)_J^\top), -\basisx(\rx_i)_\text{shift}^\top)^\top \hat{\parm}_N
\end{eqnarray*}
with $\mA$ being the matrix with rows $\basisy_1(\ry_k)$ for $k = 1, \dots, K$, 
$\mB$ the matrix with rows $(-\basisx_1(\rx_i)^\top,\dots, \basisx(\rx_i)_J^\top)$
for $i = 1, \dots, N_\text{new}$ and $\mB_\text{shift}$ the matrix with rows 
$\basisx(\rx_i)^\top$ for $i = 1, \dots, N_\text{new}$, the transformation function can
be simultaneously evaluated for all $k = 1, \dots, K$ and $i = 1, \dots, N_\text{new}$ as
\begin{eqnarray*}
(\mA \otimes \mB \mid \bold{1}_K \otimes \mB_\text{shift})^\top \hat{\parm}_N.
\end{eqnarray*}
This product is in the special form of an array model \citep{Currie_Durban_Eilers_2006} and
is computed very quickly by \pkg{mlt} using the tricks described by \cite{Currie_Durban_Eilers_2006}.


\section{Classical Likelihood Inference} \label{sec:asympt}

Because the problem of estimating an unknown distribution function is 
embedded in the maximum likelihood framework in \pkg{mlt}, the asymptotic analysis
benefits from standard results on the asymptotic behaviour of maximum
likelihood estimators.  The contribution of an ``exact continuous''
observation $\ry$ from an absolutely continuous distribution to the Fisher
information is approximately
\begin{eqnarray*}
\mF(\parm \mid \rY \in (\ubar{\ry}, \bar{\ry}]) & \approx & 
-\frac{\partial^2 \log(\dY(\ry \mid \parm))}{\partial \parm
\partial \parm^\top} \nonumber \\
& = & - \left(
\basisy(\ry) \basisy(\ry)^\top \left\{
    \frac{\dZ^{\prime\prime}(\basisy(\ry)^\top \parm)}{\dZ(\basisy(\ry)^\top \parm)}
   -\left[\frac{\dZ^{\prime}(\basisy(\ry)^\top \parm)}{\dZ(\basisy(\ry)^\top \parm)}\right]^2\right\}
   - \frac{\basisy^\prime(\ry){\basisy^\prime(\ry)}^\top}{{(\basisy^\prime(\ry)}^\top\parm)^2}\right). \label{f:F_exact}
\end{eqnarray*}
For a censored or discrete observation, we have the following
contribution to the Fisher information
\begin{eqnarray*}
\mF(\parm \mid \rY \in (\ubar{\ry}, \bar{\ry}]) & = & -\frac{\partial^2 \log(\lik(\basisy^\top \parm \mid
\rY \in (\ubar{\ry}, \bar{\ry}]))}{\partial \parm \partial \parm^\top} \nonumber \\
& = & 
- \left\{\frac{\dZ^\prime(\basisy(\bar{\ry})^\top \parm)\basisy(\bar{\ry})\basisy(\bar{\ry})^\top -
      \dZ^\prime(\basisy(\ubar{\ry})^\top \parm) \basisy(\ubar{\ry}) \basisy(\ubar{\ry})^\top}
     {\pZ(\basisy(\bar{\ry})^\top \parm) - \pZ(\basisy(\ubar{\ry})^\top \parm)}
\right. \label{f:F_interval} \\
& &  \quad -\frac{[\dZ(\basisy(\bar{\ry})^\top \parm)\basisy(\bar{\ry}) - 
       \dZ(\basisy(\ubar{\ry})^\top \parm) \basisy(\ubar{\ry})] }
     {[\pZ(\basisy(\bar{\ry})^\top \parm) - \pZ(\basisy(\ubar{\ry})^\top \parm]^2} \times \nonumber \\
& & \left. \qquad      [\dZ(\basisy(\bar{\ry})^\top \parm)\basisy(\bar{\ry})^\top - 
       \dZ(\basisy(\ubar{\ry})^\top \parm) \basisy(\ubar{\ry})^\top]
\right\}. \nonumber
\end{eqnarray*}
For a truncated observation, the Fisher information is given by
$\mF(\parm \mid \rY \in (\ubar{\ry}, \bar{\ry}]) - \mF(\parm \mid \rY \in (\ry_l,
\ry_r])$.

Based on these results, we can construct asymptotically valid confidence
intervals and confidence bands for the conditional distribution function
from confidence intervals and bands for the linear functions $\basisy^\top
\parm$. 

\paragraph{Categorical Data Analysis: Chinese Survey (Cont'd)}

For the proportional odds model for happiness given age and income, we compare the score function 
\citep[using the \cmd{estfun} method from package \pkg{sandwich},][]{Zeileis_2004,pkg:sandwich}
by their relative change
<<mlt-CHFLS-2-cmpr-estfun>>=
sc_polr <- estfun(polr_CHFLS_2)
sc_mlt <- -estfun(mlt_CHFLS_2)[,c(4, 5, 1:3)]
summary((sc_polr - sc_mlt) / 
        pmax(sqrt(.Machine$double.eps), sc_mlt))
@
and the standard errors of the
regression coefficients
<<mlt-CHFLS-2-cmpr-2>>=
RC(polr = sqrt(diag(vcov(polr_CHFLS_2))),
   mlt = sqrt(diag(vcov(mlt_CHFLS_2)))[c(4, 5, 1:3)])
@
The positive effect of income is ``significant'' by standard measures
\citep[the classical coefficient table was obtained using \cmd{cftest} from package 
\pkg{multcomp},][]{Hothorn_Bretz_Westfall_2008,pkg:multcomp}
<<mlt-CHFLS-2-cmpr-3>>=
cftest(polr_CHFLS_2)
cftest(mlt_CHFLS_2, parm = names(coef(polr_CHFLS_2)))
@

\paragraph{Survival Analysis: GBSG-2 Trial (Cont'd)}

For the ``classical'' Cox model \code{mlt_GBSG2_1}, the standard errors
of all three methods applied (\cmd{coxph}, \cmd{mlt}, \cmd{flexsurvspline})
are more or less identical 
<<mlt-GBSG2-1-coxph-cmpr>>=  
cf <- coef(coxph_GBSG2_1)
RC(coxph = sqrt(diag(vcov(coxph_GBSG2_1))),
   mlt = sqrt(diag(vcov(mlt_GBSG2_1)))[names(cf)],
   fss = sqrt(diag(vcov(fss_GBSG2_1)))[names(cf)])
@
As a consequence, the corresponding coefficient tables, here produced
using \cmd{cftest}, are also rather similar
<<mlt-GBSG2-1-coxph-cmpr-cftest>>=  
cftest(coxph_GBSG2_1)
cftest(mlt_GBSG2_1, parm = names(cf))
cftest(fss_GBSG2_1, parm = names(cf))
@

\paragraph{Density Estimation: Geyser Data (Cont'd)}

A relatively simple method for the construction of asymptotic confidence bands
is based on the multivariate joint normal distribution of the model coefficients
$\hat{\parm}_N$. A confidence band for the unconditional transformation function 
of waiting times is derived from simultaneous confidence intervals for the 
linear function $\mA \hat{\parm}_N$ as implemented in package \pkg{multcomp}. Here $\mA$ is
the matrix of Bernstein basis functions evaluated on a grid of \code{K} waiting times
$\ry_1, \dots, \ry_K$. The quantile adjusted for multiplicity is then used
on a finer grid of \code{cheat} values for plotting purposes
<<mlt-geyser-w-band>>=
cb_w <- confband(mlt_w, newdata = data.frame(1), K = 20, cheat = 100)
@
The result is shown in Figure~\ref{fig:geyser-w-cbplot} on the scale of the
transformation and distribution function.


\begin{figure}
\begin{center}
<<mlt-geyser-w-cbplot, echo = FALSE>>=
layout(matrix(1:2, ncol = 2))
#i <- (cb_w[, "q"] > 45 & cb_w[, "q"] < 110)
#cb_w[-i, "lwr"] <- NA
#cb_w[-i, "upr"] <- NA
plot(cb_w[, "q"], cb_w[, "Estimate"], xlab = "Waiting times", ylab = "Transformation", 
     main = "", type = "l")

q <- cb_w[, "q"]
lwr <- cb_w[, "lwr"]
upr <-  cb_w[, "upr"]
polygon(c(q, rev(q)), c(lwr, rev(upr)),
        border = NA, col = "lightblue") ### rgb(.1, .1, .1, .1))

lines(cb_w[, "q"], cb_w[, "Estimate"])
rug(geyser$waiting, col = rgb(.1, .1, .1, .1))
plot(cb_w[, "q"], pnorm(cb_w[, "Estimate"]), xlab = "Waiting times", ylab = "Distribution", 
     main = "", cex = .5, type = "n")
polygon(c(q, rev(q)), c(pnorm(lwr), rev(pnorm(upr))),                                     
        border = NA, col = "lightblue") ### rgb(.1, .1, .1, .1))
lines(cb_w[, "q"], pnorm(cb_w[, "Estimate"]))

# lines(cb_w[, "q"], pnorm(cb_w[, "lwr"]))
# lines(cb_w[, "q"], pnorm(cb_w[, "upr"]))
rug(geyser$waiting, col = rgb(.1, .1, .1, .1))
@
\caption{Old Faithful Geyser. Estimated transformation (left)
         and distribution functions (right) with asymptotic confidence bands.
         \label{fig:geyser-w-cbplot}}
\end{center}
\end{figure}


\section{Simulation-based Likelihood Inference} \label{sec:sim}

The fully specified probabilistic model $(\pZ, \basisy, \hat{\parm}_N)$
allows sampling from the distribution $\hat{\Prob}_\rY$.
For estimated parameters $\hat{\parm}_N$, this model-based or 
``parametric'' bootstrap from $\hat{\Prob}_\rY$ can be implemented by the
probability integral transform, \ie $\rZ_1, \dots, \rZ_N \stackrel{\text{iid}}{\sim} \Prob_\rZ$ is
drawn and then $\rY_i^\star = \inf\{\ry \in \samY \mid \basisy(\ry)^\top \hat{\parm}_N \ge
\rZ_i\}$ is determined by numerical inversion of the distribution function.
The \cmd{simulation} method for \code{mlt} objects first computes the
distribution function over a grid of \code{K} response values, draws \code{nsim} times
\code{nrow(newdata)} random samples from $\Prob_\rZ$ and returns 
the intervals $\ry_k < \rY_i^\star < \ry_{k + 1}$ (\code{interpolate = FALSE}) or
a linear interpolation thereof (\code{interpolate = TRUE}). 


\paragraph{Density Estimation: Geyser Data (Cont'd)}

The model-based bootstrap analogue of the confidence band for the
distribution function of waiting times (Figure~\ref{fig:geyser-w-cbplot},
right panel) based on $100$ bootstrap samples is generated from the
following code.  First, $100$ samples of size $N = \Sexpr{nrow(geyser)}$ are
drawn from the fitted unconditional model \code{mlt\_w} for waiting times.  
For each sample, the model \code{ctm_w} is refitted, using the
parameters $\hat{\parm}_N$ as starting values (\code{theta}) to speed-up the
computations.  Next, the log-likelihood ratio
\begin{eqnarray*}
\sum_{i = 1}^N \log(\lik(\basisy^\top \hat{\parm}^\star_N \mid \rY_i^\star) - 
\sum_{i = 1}^N \log(\lik(\basisy^\top \hat{\parm}_N \mid \rY_i^\star)
\end{eqnarray*}
is computed for each sample. Last, distribution and density functions are obtained
for each of the models in this small loop
<<mlt-geyser-w-simulate, results = "hide", cache = TRUE>>=
new_w <- simulate(mlt_w, nsim = 100)
llr <- numeric(length(new_w))
pdist <- vector(mode = "list", length = length(new_w))
pdens <- vector(mode = "list", length = length(new_w))
ngeyser <- geyser
q <- mkgrid(var_w, 100)[[1]]
for (i in 1:length(new_w)) {
    ngeyser$waiting <- new_w[[i]]
    mlt_i <- mlt(ctm_w, data = ngeyser, scale = TRUE, 
                 theta = coef(mlt_w))
    llr[[i]] <- logLik(mlt_i) - logLik(mlt_i, parm = coef(mlt_w))
    pdist[[i]] <- predict(mlt_i, newdata = data.frame(1), 
                          type = "distribution", q = q)
    pdens[[i]] <- predict(mlt_i, newdata = data.frame(1), 
                          type = "density", q = q)
}
@

The distribution and density functions corresponding to log-likelihood ratios less then the $95\%$
quantile of the $100$ log-likelihood ratios, \ie after removal of five extreme
curves, are plotted in Figure~\ref{geyser-w-simulate-plot} and can be
interpreted as a band around the distribution and density function.  The
asymptotic band for the distribution function nicely fits the band obtained
from the bootstrap sample but the latter does not deteriorate for probabilities close
to zero and one.

In the conditional case, \code{nsim} samples from the $N$ conditional distributions
$\hat{\Prob}_{\rY \mid \rX = \rx_i}, i = 1, \dots, N$ are drawn by \cmd{simulate}. 

\begin{figure}
\begin{center}
<<mlt-geyser-w-simulate-plot, echo = FALSE>>=
i <- which(llr < quantile(llr, prob = .95))
tpdist <- pdist[i]
tpdens <- pdens[i]
layout(matrix(1:2, ncol = 2))
plot(q, tpdist[[1]], type = "n", xlab = "Waiting times", ylab = "Distribution")
polygon(c(q, rev(q)), c(pnorm(lwr), rev(pnorm(upr))),
        border = NA, col = "lightblue")
tmp <- sapply(tpdist, function(x) lines(q, x, col = rgb(.1, .1, .1, .1)))
plot(q, tpdens[[1]], type = "n", ylim = range(unlist(pdens)),
     xlab = "Waiting times", ylab = "Density")
tmp <- sapply(tpdens, function(x) lines(q, x, col = rgb(.1, .1, .1, .1)))
@
\caption{Old Faithful Geyser. Model-based bootstrap for distribution (left, the blue area depicts the
         asymptotic confidence band from Figure~\ref{fig:geyser-w-cbplot}) and density (right) function for 
         the unconditional transformation model parameterised in terms of a Bernstein polynomials
         of order $8$. \label{geyser-w-simulate-plot}}
\end{center}
\end{figure}

\section{Summary}

The computational framework implemented in package \pkg{mlt} allows fitting of
a large class of transformation models.  Many established \proglang{R}
add-on packages implement special cases, mostly in the survival analysis context.
The most prominent one is the \pkg{survival} package
\citep{Therneau_Grambsch_2000,pkg:survival} with partial likelihood
estimation of the Cox model in \cmd{coxph} and parametric linear transformation models
in \cmd{survreg}.  Parametric proportional hazards (\cmd{phreg}) and accelerated failure time
models are also available from package \pkg{eha} \citep{pkg:eha}.
The results obtained using these functions are
practically identical to those obtained from the unified implementation of
transformation models in \pkg{mlt} as shown in the various examples
presented in Section~\ref{sec:trafo}.  Other packages offer estimation of
the Cox model for interval-censored responses, for example 
\pkg{coxinterval} \citep{pkg:coxinterval}, \pkg{MIICD}
\citep{pkg:MIICD} and \pkg{ICsurv} \citep{pkg:ICsurv}.  No special treatment
of this situation is necessary in \pkg{mlt} as the likelihood maximised by
\cmd{mlt} allows arbitrary schemes of random censoring and also truncation.

Alternative likelihood approaches to transformation models often
parameterise the hazard or log-hazard function by some spline, including the
\proglang{R} add-on packages \pkg{polspline} \citep{pkg:polspline},
\pkg{logspline} \citep{pkg:logspline}, \pkg{bshazard} \citep{pkg:bshazard}
and \pkg{gss} \citep{Gu_2014,pkg:gss}.  Packages \pkg{muhaz}
\citep{pkg:muhaz} and \pkg{ICE} \citep{pkg:ICE} implement kernel smoothing
for hazard function estimation, the latter package allows for
interval-censored responses.  The penalised maximum likelihood estimation
procedure for simultaneous estimation of the baseline hazard function and
the regression coefficients in a Cox model is available from package
\pkg{survivalMPL} \citep{pkg:survivalMPL}.  Estimation of the unconstrained log-hazard 
function is theoretically attractive but too erratic estimates have to be dealt with by
some form of penalisation. A direct parameterisation of the transformation
function, \ie the log-cumulative baseline hazard in the Cox model, only requires
monotone increasing functions to be fitted. Thus, penalisation is not necessary
but one has to deal with a constrained problem. Package \pkg{mlt} follows the 
latter approach.

Based on the estimation equation procedure by \cite{Chenetal_2002},
\pkg{TransModel} \citep{pkg:TransModel} implements continuous time
proportional hazards and proportional odds linear transformation models. 
Time-varying coefficients can be estimated using packages \pkg{dynsurv}
\citep{pkg:dynsurv} and \pkg{timereg}
\citep{Scheike_Martinussen_2006,Scheike_Zhang_2011,pkg:timereg}
Discrete proportional odds or proportional hazards models for the analysis
of ordered categorical responses are implemented in packages
\pkg{MASS} \citep{Venables_Ripley_2002, pkg:MASS}, \pkg{ordinal} \citep{pkg:ordinal}
and \pkg{VGAM} \citep{Yee_2010, pkg:VGAM}. 

Maximum likelihood estimators for a fair share of the models implemented in
these established packages can be re-implemented using
the computational infrastructure offered by package \pkg{mlt}.  The
availability of (at least) two independent implementations allows package
developers to validate their implementation.  In fact, some errors in
earlier development versions of \pkg{mlt} could be detected by comparing
model outputs.  Because the implementation of maximum likelihood estimation
for conditional transformation models presented in this paper relies on a
rather dense code base ($200$ lines of pure \proglang{R} code in
\pkg{variables}, $860$ lines in \pkg{basefun} and $1450$ lines in \pkg{mlt},
all convenience functions and user interfaces included), the likelihood of
implementation errors is smaller compared the likelihood of errors in any of
the plethora of alternative implementations of special linear transformation
models (but not zero, of course).

The modelling abilities of \pkg{mlt} go beyond what is currently available
in \proglang{R}.  Maybe the practically most interesting example is
distribution regression, \ie transformation models with response-varying
regression coefficients.  The only special case available in \proglang{R} we
are aware of are Cox models with time-varying effects in packages
\pkg{dynsurv} and \pkg{timereg}.  For other models, such as the distribution
regression analysis of the Boston Housing data presented here, or for
non-proportional odds or non-proportional hazards models, implementations
are lacking.  Our analysis of the bird counts example is novel also from a
modelling point of view as linear transformation models for count data are
still waiting to be kissed awake.

One unique feature of \pkg{mlt} is the strict separation of model specification
(using \cmd{ctm}) and model estimation (using \cmd{mlt}) allowing
computations on unfitted models.  Models are specified by combinations of
basis functions instead of using the rather restrictive formula language. 
In addition, model coefficients can be altered by the user, for example for
computing conditional distribution or density functions or for the
evaluation of the log-likelihood at arbitrary values of the parameters.  The
\cmd{simulate} method for \code{mlt} objects can always be used to draw
samples from fitted (or unfitted) transformation models.  Thus, the
implementation of parametric bootstrap procedures is a straightforward
exercise.

The very flexible and powerful user interface of \pkg{mlt} will, however, be
incomprehensible for most of its potential users.  Because transformation
models seldomly receive the attention they deserve in statistics courses,
the unorthodox presentation of regression models ignoring the fences between
the traditionally compartmentalised fields of ``regression analysis'',
``survival analysis'', or ``categorical data analysis'' in this
documentation of \pkg{mlt} will likely also confuse many statisticians, let
alone data or subject-matter scientists.  Current efforts concentrate on the
implementation of convenience interfaces, \ie higher-level user interfaces
for special forms of transformation models, which resemble the traditional
notational and naming conventions from the statistical modelling literature. 
Standard formula-based interfaces to the underlying \pkg{mlt} implementation
of Cox models, parametric survival models, models for ordered categorical
data, normal linear and non-normal linear models \citep[including \cmd{Colr}
implementing continuous outcome logistic
regression,][]{Lohse_Rohrmann_Faeh_2017} are available in package \pkg{tram}
\citep{pkg:tram}.  The corresponding package vignette demonstrates how some
of the model outputs presented in this paper can be obtained in simpler
ways.

The core functionality provided by \pkg{mlt} was instrumental in developing
statistical learning procedures based on transformation models. 
Transformation trees and corresponding transformation forests were
introduced by \cite{Hothorn_Zeileis_2017} and implemented in package
\pkg{trtf}; an application to conditional distributions for body mass
indices was described by \cite{Hothorn_2018_SM} and novel survival forests have
been evaluated by \cite{Korepanova_Seibold_Steffen_2019}.  Two different
gradient boosting schemes allowing complex models to be built in the
transformation modelling framework were proposed by \cite{Hothorn_2019} and
are implemented in package \pkg{tbm}.

\bibliography{mlt,packages}

\newpage

\begin{appendix}

\section*{Appendix}

\section[The variables Package]{The \pkg{variables} Package} \label{app:variables}

The \pkg{variables} package \citep{pkg:variables} offers a small collection
of classes and methods for specifying and computing on abstract variable
descriptions.  The main purpose is to allow querying properties of variables
without having access to observations.  A variable description allows to
extract the name (\cmd{variable.name}), description (\cmd{desc}) and unit
(\cmd{unit}) of the variable along with the support (\cmd{support}) and
possible bounds (\cmd{bounds}) of the measurements.  The \cmd{mkgrid} method
generates a grid of observations from the variable description.  The package
differentiates between factors, ordered factors, discrete, and continuous
numeric variables.

\subsection{Unordered Factors}

We use eye color as an example of an unordered factor. The corresponding
variable description is defined by the name, description and levels of this
factor
<<mlt-variables-factor>>=
f_eye <- factor_var("eye", desc = "eye color", 
                    levels = c("blue", "brown", "green", "grey", "mixed"))
@
The properties of this factor are
<<mlt-variables-factor-methods>>=
variable.names(f_eye)
desc(f_eye)
variables::unit(f_eye)
support(f_eye)
bounds(f_eye)
is.bounded(f_eye)
@
and we can generate values, \ie an instance of this factor with unique levels, via
<<mlt-variables-factor-mkgrid>>=
mkgrid(f_eye)
@

\subsection{Ordered Factors}

An ordered factor, temperature in categories is used here as an example, is defined as
in the unordered case
<<mlt-variables-ordered>>=
o_temp <- ordered_var("temp", desc = "temperature", 
                      levels = c("cold", "lukewarm", "warm", "hot"))
@
and the only difference is that explicit bounds are known
<<mlt-variables-ordered-methods>>=
variable.names(o_temp)
desc(o_temp)
variables::unit(o_temp)
support(o_temp)
bounds(o_temp) 
is.bounded(o_temp)
mkgrid(o_temp)
@

\subsection{Discrete Numeric Variables}

Discrete numeric variables are defined by \cmd{numeric\_var} with integer-valued
\code{support} argument, here using age of a patient as example
<<mlt-variables-fd>>=
v_age <- numeric_var("age", desc = "age of patient", 
                     unit = "years", support = 25:75)
@
The variable is bounded with finite support
<<mlt-variables-fd-methods>>=
variable.names(v_age)
desc(v_age)
variables::unit(v_age)
support(v_age)
bounds(v_age) 
is.bounded(v_age)
@
and the support is returned in
<<mlt-variables-fd-mkgrid>>=
mkgrid(v_age)
@

\subsection{Continuous Numeric Variables}

For conceptually continuous variables the \code{support} argument is a
double vector with two elements representing an interval acting as the
support for any basis function to be defined later.  The variable may or may
not be bounded ($\pm \infty$ is allowed).  For generating equidistant 
grids, \code{support + add} is used for unbounded variables or the
corresponding finite boundaries if \code{add} is zero.  Daytime temperature
at Zurich, for example, could be presented as
<<mlt-variables-c>>=
v_temp <- numeric_var("ztemp", desc = "Zurich daytime temperature", 
                      unit = "Celsius", support = c(-10.0, 35.0), 
                      add = c(-5, 5), bounds = c(-273.15, Inf))
@
Basis functions for this variable shall be defined 
for temperatures between $-10$ and $35$ degrees Celsius
<<mlt-variables-c-methods>>=
variable.names(v_temp)
desc(v_temp)
variables::unit(v_temp)
support(v_temp)
bounds(v_temp) 
is.bounded(v_temp)
@
One might be interested in evaluating model predictions outside \code{support} as
defined by the \code{add} argument, so \cmd{mkgrid} generates a equidistant grid
between $-15$ and $40$ degrees Celsius
<<mlt-variables-c-mkgrid>>=
mkgrid(v_temp, n = 20)
@

\subsection{Multiple Variables}

We can join multiple variable descriptions via \cmd{c}
<<mlt-variables-vars>>=
vars <- c(f_eye, o_temp, v_age, v_temp)
@
and all methods discussed above work accordingly
<<mlt-variables-vars-methods>>=
variable.names(vars)
desc(vars) 
variables::unit(vars)
support(vars)
bounds(vars)
is.bounded(vars)
mkgrid(vars, n = 20)
@
Calling
<<mlt-variables-vars-expand>>=
nd <- expand.grid(mkgrid(vars))
@
generates a \code{data.frame} with all possible values of the
variables and all combinations thereof. The generic \cmd{as.vars}
takes a data frame of observations as input and derives abstract
variable descriptions using the available information. The generic
function \cmd{check} returns \code{TRUE} if \code{data} matches
the abstract description
<<mlt-variables-check>>=
check(vars, data = nd)
@


\section[The basefun Package]{The \pkg{basefun} Package} \label{app:basefun}

The \pkg{basefun} package \citep{pkg:basefun} implements Bernstein,
Legendre, log and polynomial basis functions.  In addition, facilities for
treating arbitrary model matrices as basis functions evaluated on some data
are available.  Basis functions can be joined column-wise using \cmd{c} or
the box product can be generated using \cmd{b}.  The definition of basis
functions does not require any actual observations, only variable
descriptions (see Appendix~\ref{app:variables}) are necessary.  Each basis
offers \cmd{model.matrix} and \cmd{predict} methods.  We illustrate how one
can set-up some of these basis functions in the following sections.

\subsection{Polynomial Basis}

For some positive continuous variable $x$ we want to deal with the polynomial
$\alpha + \beta_1 x + \beta_3 x^3$ and first set-up a variable description
(see Appendix~\ref{app:variables}) and the basis function
(we set-up a basis of order three ommiting the quadratic term)
<<mlt-basefun-polynom>>=
xvar <- numeric_var("x", support = c(0.1, pi), bounds= c(0, Inf))
x <- as.data.frame(mkgrid(xvar, n = 20))
class(pb <- polynomial_basis(xvar, coef = c(TRUE, TRUE, FALSE, TRUE)))
@
The basis function \code{pb} is a \code{function}, therefore we can
evaluate the basis as
<<mlt-basefun-polynom-fun>>=
head(pb(x))
@
or, equivalently, using the corresponding \cmd{model.matrix} method (which is preferred)
<<mlt-basefun-polynom-mm>>=
head(model.matrix(pb, data = x))
@
Evaluating the polynomial for some coefficients is done by the
\cmd{predict} method
<<mlt-basefun-polynom-pred>>=
predict(pb, newdata = x, coef = c(1, 2, 0, 1.75))
@
which also allows derivatives to be computed
<<mlt-basefun-polynom-pred-deriv>>=
predict(pb, newdata = x, coef = c(1, 2, 0, 1.75), deriv = c(x = 1L))
@

\subsection{Logarithmic Basis}

The monotone increasing logarithmic basis $\eparm_1 + \eparm_2 \log(x)$ being 
subject to $\eparm_2 > 0$ is defined as
<<mlt-basefun-log>>=
class(lb <- log_basis(xvar, ui = "increasing"))
head(X <- model.matrix(lb, data = x))
@
The model matrix contains a \code{constraint} attribute 
<<mlt-basefun-log-constr>>=
attr(X, "constraint")
@
where the linear constraints $\mA \parm \ge \mvec$ are represented by
a matrix $\mA$ (\code{ui}) and a vector $\mvec$ (\code{ci}). For 
$\parm = (1, 2)$ the function and its derivative can be computed
as
<<mlt-basefun-log-pred>>=
predict(lb, newdata = x, coef = c(1, 2))
predict(lb, newdata = x, coef = c(1, 2), deriv = c(x = 1L))
@

\subsection{Bernstein Basis}
A monotone increasing Bernstein polynomial $\bern{3}$ can 
be defined and evaluated as
<<mlt-basefun-Bernstein>>=
class(bb <- Bernstein_basis(xvar, order = 3, ui = "increasing"))
head(X <- model.matrix(bb, data = x))
@
We can check if the constraints are met for some parameters
<<mlt-basefun-Bernstein-constr>>=
cf <- c(1, 2, 2.5, 2.6)
(cnstr <- attr(X, "constraint"))
all(cnstr$ui %*% cf > cnstr$ci)
@
where a \code{Matrix} object \citep[from package \pkg{Matrix},][]{pkg:Matrix} represents
the linear constraints.
Other possible constraints include a decreasing function (\code{ui = "decreasing"}), 
positive and negative functions (\code{ui = "positive"} or \code{ui = "negative"}),
as well as cyclic or integral zero functions (\code{ui = "cyclic"} or \code{ui = "zerointegral"}).
The polynomial defined by the basis functions and parameters and its first
derivative can be evaluated using \cmd{predict}
<<mlt-basefun-Bernstein-predict>>=
predict(bb, newdata = x, coef = cf)
predict(bb, newdata = x, coef = cf, deriv = c(x = 1))
@

\subsection{Model Matrices}

Model matrices are basis functions evaluated at some data. The \pkg{basefun}
package offers an \cmd{as.basis} method for \code{formula} objects which
basically represents unevaluated calls to \cmd{model.matrix} with two
additional arguments (\code{remove_intercept} removes the intercept
\textit{after} appropriate contrasts were computed and \code{negative}
multiplies the model matrix with $-1$).  Note that the \code{data} argument
does not need to be a \code{data.frame} with observations, a variable
description is sufficient. If \code{data} is a data frame of observations,
\code{scale = TRUE} scales each columns of the model matrix to the unit interval. Here is
an example using the iris data
<<mlt-basefun-as.basis>>=
iv <- as.vars(iris)
fb <- as.basis(~ Species + Sepal.Length + Sepal.Width,  data = iv,
               remove_intercept = TRUE, negative = TRUE, 
               contrasts.args =  list(Species = "contr.sum"))
class(fb)
head(model.matrix(fb, data = iris))
@

\subsection{Combining Bases}

Two (or more) basis functions can be concatenated by simply joining the corresponding
model matrices column-wise. If constraints $\mA_i \parm_i \ge \mvec_i$ are present for $i = 1, 2, \dots$ 
the overall constraints are given by a block-diagonal matrix $\mA = \text{blockdiag}(\mA_1, \mA_2, \dots)$,
$\parm = (\parm_1^\top, \parm_2^\top, \dots)^\top$ and $\mvec = (\mvec_1^\top, \mvec_2^\top, \dots)^\top$. As
an example we add a positive log-transformation to a Bernstein polynomial
<<mlt-basefun-c>>=
class(blb <- c(bern = bb, 
               log = log_basis(xvar, ui = "increasing", 
                               remove_intercept = TRUE)))
head(X <- model.matrix(blb, data = x))
attr(X, "constraint")
@

The box product of two basis functions is defined by the row-wise Kronecker product of the
corresponding model matrices but \textit{not} the Kronecker product of the model matrices (which
would result in a matrix with the same number of columns but squared number of rows).
The following definition leads to one intercept and one deviation function
<<mlt-basefun-b>>=
fb <- as.basis(~ g, data = factor_var("g", levels = LETTERS[1:2]))
class(bfb <- b(bern = bb, f = fb))
nd <- expand.grid(mkgrid(bfb, n = 10))
head(X <- model.matrix(bfb, data = nd))
attr(X, "constraint")
@
If \code{sumconstr = TRUE}, only the sum of the two functions
is expected to meet the monotonicity constraint
<<mlt-basefun-b-sumconstr>>=
bfb <- b(bern = bb, f = fb, sumconstr = TRUE)
head(X <- model.matrix(bfb, data = nd))
attr(X, "constraint")
@

\section[The mlt Package]{The \pkg{mlt} Package} \label{app:mlt}

\subsection{Specifying Response Observations} \label{subapp:R}

The generic function \cmd{R} is an extention of \cmd{Surv} (package \pkg{survival}) to possibly 
truncated, categorical or integer-valued response variables with methods for objects of class 
\code{Surv}, \code{ordered}, \code{factor}, \code{integer} and \code{numeric}. The main
purpose is to set-up the interval $(\ubar{\ry}, \bar{\ry}]$ for the evaluation
of the likelihood contribution $\pZ(\h(\bar{\ry})) - \pZ(\h(\ubar{\ry}))$.
For ordered categorical responses, $\bar{\ry}$ is the observed level and
$\ubar{\ry}$ the level preceding $\bar{\ry}$ (which is missing when the
first or last level was observed)
<<mlt-R-ordered>>=
head(R(sort(unique(CHFLS$R_happy))))
@
For integers, $\bar{\ry}$ is the observation and $\ubar{\ry} = \bar{\ry} - 1$
<<mlt-R-integer>>=
R(1:5, bounds = c(1, 5))
@
Numeric observations can be ``exact'' (meaning that the likelihood is
approximated by the density evaluated at the observation), interval-censored
(with left- and right-censoring as special cases) and left- or right-truncated
in arbitrary combinations thereof. For example, consider ten draws from
the standard normal, truncated to $(-1, 2]$ and possibly censored
<<mlt-R-numeric>>=
x <- rnorm(10)
xt <- round(x[x > -1 & x <= 2], 3)
xl <- xt - sample(c(Inf, (0:(length(xt) - 2)) / length(xt)), 
                  replace = FALSE)
xr <- xt + sample(c(Inf, (0:(length(xt) - 2)) / length(xt)), 
                  replace = FALSE)
R(c(1.2, rep(NA, length(xt))), cleft = c(NA, xl), cright = c(NA, xr), 
  tleft = -1, tright = 2)
@
Censoring and truncation (via the \code{cleft}, \code{cright}, \code{tleft}
and \code{tright} arguments) are also implemented for ordered categorical
and integer responses. \code{Surv} objects are simply converted to the 
alternative format without the possibility to define extra arguments
<<mlt-R-Surv>>=
head(geyser$duration)
head(R(geyser$duration))
@
It should be noted that the measurement scale of the observations has
nothing to do with the measurement scale of the response $\rY$ in the model
and the corresponding basis functions.  Discrete or continuous models are
specified based on the abstract variable description
(Appendix~\ref{app:variables}).

\subsection{Methods for Conditional Transformation Models}

Methods for the generic functions \cmd{bounds},
\cmd{variable.names}, \cmd{model.matrix}, \cmd{mkgrid}
are available for abstract model descriptions in \code{ctm}
objects are returned by \cmd{ctm}.

\subsection{Methods for Fitted Transformation Models}

For transformation models fitted using \cmd{mlt}, the following methods are
available: \cmd{update} (for refitting the model), \cmd{logLik} (the
log-likelihood, optionally also as a function of parameters $\parm$), \cmd{estfun} (the
scores), \cmd{coef} and \cmd{vcov} (model parameters and their covariance),
\code{predict} (model predictions at various scales), \cmd{confband}
(confidence bands for the transformation and distribution function),
\cmd{simulate} (sampling from the model) as well as \cmd{print},
\cmd{summary} and \cmd{plot}.  In addition, \cmd{variable.names},
\cmd{mkgrid} and \cmd{bounds} are also implemented. The 
\cmd{coef<-} method changes the parameters in the model. One last example,
the approximation of a $\chi^2_{20}$ distribution by an unconditional
transformation model, shall illustrate how one interacts with \code{ctm} and 
\code{mlt} objects. We first define the ``true'' distribution
<<mlt-mlt-chi-p>>=
pY <- function(x) pchisq(x, df = 20)
dY <- function(x) dchisq(x, df = 20)
qY <- function(p) qchisq(p, df = 20)
@
and set-up a Bernstein polynomial for the transformation function
<<mlt-mlt-chi-B>>=
yvar <- numeric_var("y", support = qY(c(.001, 1 - .001)), 
                    bounds = c(0, Inf))
By <- Bernstein_basis(yvar, order = ord <- 15, ui = "increasing")
@
The \emph{unfitted} model is now
<<mlt-mlt-chi-mlt>>=
mod <- ctm(By)
@
and we compute the true transformation function
<<mlt-mlt-chi-trafo>>=
h <- function(x) qnorm(pY(x))
x <- seq(from = support(yvar)[["y"]][1], to = support(yvar)[["y"]][2], 
         length.out = ord + 1)
@
and set the parameters of the model using
<<mlt-mlt-chi-coef>>=
mlt::coef(mod) <- h(x)
@
We can now simulate from this \emph{purely theoretical} model
<<mlt-mlt-chi-sim>>=
d <- as.data.frame(mkgrid(yvar, n = 500))
d$grid <- d$y
d$y <- simulate(mod, newdata = d)
@
fit this model to the simulated data
<<mlt-mlt-chi-fit>>=
fmod <- mlt(mod, data = d, scale = TRUE)
@
and compare the true (\code{mod}) and fitted
(\code{fmod}) models by looking at the coefficients
and log-likelihoods (evaluated for the estimated and
true coefficients)
<<mlt-mlt-chi-model>>=
coef(mod)
coef(fmod)
logLik(fmod)
logLik(fmod, parm = coef(mod))
@
The corresponding density functions of the $\chi^2_{20}$ distribution, the
approximating true transformation model \code{mod} and the estimated transformation
model \code{fmod} are plotted in Figure~\ref{fig:mlt-chi-plot} using the
code shown on top of the plot.

\begin{figure}[h!]
\begin{center}
<<mlt-mlt-chi-plot>>=
## compute true density
d$dtrue <- dY(d$grid)
d$dest <- predict(fmod, q = sort(d$grid), type = "density")
plot(mod, newdata = d, type = "density", col = "black", 
     xlab = "y", ylab = "Density", ylim = c(0, max(d$dest)))
lines(d$grid, d$dtrue, lty = 2)
lines(sort(d$grid), d$dest[order(d$grid)], lty = 3)
legend("topright", lty = 1:3, bty = "n", 
       legend = c("True", "Approximated", "Estimated"))
@
\caption{$\chi^2_{20}$ Data. Density of a $\chi^2_{20}$ (true), 
         an approximating unconditional transformation model and
         the density fitted to a sample from the approximating model.
         \label{fig:mlt-chi-plot}}
\end{center}
\end{figure}



%\section{Computational Details}

<<mlt-funs, echo = FALSE, results='hide', purl = FALSE, cache = FALSE>>=
if (file.exists("packages.bib")) file.remove("packages.bib")
pkgversion <- function(pkg) {
    pkgbib(pkg)
    packageDescription(pkg)$Version
}
pkgbib <- function(pkg) {
    x <- citation(package = pkg, auto = TRUE)[[1]]
    b <- toBibtex(x)
    b <- gsub("Buehlmann", "B{\\\\\"u}hlmann", b)

    i <- grep("title = \\{ICsurv", b)
    if (length(i) == 1)
       b[i] <- "  title = {ICsurv: A Package for Semiparametric Regression Analysis of Interval-censored Data},"

    i <- grep("title = \\{dynsurv", b)
    if (length(i) == 1)
       b[i] <- "  title = {dynsurv: Dynamic Models for Survival Data},"

    i <- grep("title = \\{np", b)
    if (length(i) == 1)
       b[i] <- "  title = {np: Nonparametric Kernel Smoothing Methods for Mixed Data Types},"

    i <- grep("Kenneth Hess", b)
    if (length(i) == 1)
       b[i] <- "  author = {Kenneth Hess and Robert Gentleman},"

    b <- gsub("with contributions from", "and", b)

    b <- gsub("Göran Broström", "G\\\\\"oran Brostr\\\\\"om", b)

    b <- gsub(" The publishers web page is", "},", b)
    b <- gsub("http://www.crcpress.com/product/isbn/9781584885740},", "", b)

    b <- gsub("R package", "\\\\proglang{R} package", b)

    b[1] <- paste("@Manual{pkg:", pkg, ",", sep = "")
    if (is.na(b["url"])) {
        b[length(b)] <- paste("   URL = {http://CRAN.R-project.org/package=",
                              pkg, "}", sep = "")
        b <- c(b, "}")
    }
    cat(b, sep = "\n", file = "packages.bib", append = TRUE)
}
pkg <- function(pkg)
    paste("\\\\pkg{", pkg, "} \\\\citep[version~",
          pkgversion(pkg), ",][]{pkg:", pkg, "}", sep = "")

pkgs <- c("mlt", "variables", "basefun", "survival", "eha", "prodlim", "truncreg", "MASS", "nnet", "HSAUR3", "sandwich", 
  "flexsurv", "multcomp", "mgcv", "coxinterval", "ICsurv",
  "polspline", "logspline", "bshazard", "gss", "MIICD", "TransModel",
  "dynsurv", "timereg", "survivalMPL", "np", "gamlss", "Matrix", "muhaz", "ICE",
  "ordinal", "VGAM", "BB", "alabama", "tram")
out <- sapply(pkgs, pkg)

x <- readLines("packages.bib")
for (p in pkgs)
    x <- gsub(paste("\\{", p, ":", sep = ""), paste("\\{\\\\pkg{", p, "}:", sep = ""), x)
cat(x, sep = "\n", file = "packages.bib", append = FALSE)
@


\end{appendix}


<<mlt-mlt-coef, echo = FALSE, results = "hide">>= 
### print coefs for regression tests
objs <- ls()
mltobj <- objs[grep("^mlt_", objs)]
sapply(mltobj, function(m) eval(parse(text = paste("coef(", m, ")"))))
@

<<mlt-sessionInfo, echo = FALSE, results = "hide">>=
sessionInfo()
@

\clearpage

\end{document}
