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<<setup, echo=FALSE, results='hide', message = FALSE, warnings = FALSE>>=
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library("partykit")
options(prompt = "R> ", continue = "+  ", digits = 4, useFancyQuotes = FALSE)
pkgs <- c("AER", "Formula", "mlbench", "sandwich", "strucchange",
          "survival", "TH.data", "vcd", "psychotools", "psychotree", "knitr")
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@

%\VignetteEngine{knitr::knitr}
%\VignetteIndexEntry{Parties, Models, Mobsters: A New Implementation of Model-Based Recursive Partitioning in R}
%\VignetteDepends{AER,Formula,mlbench,sandwich,strucchange,survival,TH.data,vcd,psychotools,psychotree,knitr}
%\VignetteKeywords{parametric models, object-orientation, recursive partitioning}
%\VignettePackage{partykit}

\author{Achim Zeileis\\Universit\"at Innsbruck \And
        Torsten Hothorn\\Universit\"at Z\"urich}
\Plainauthor{Achim Zeileis, Torsten Hothorn}

\title{Parties, Models, Mobsters: A New Implementation of Model-Based Recursive Partitioning in \proglang{R}}
\Plaintitle{Parties, Models, Mobsters: A New Implementation of Model-Based Recursive Partitioning in R}
\Shorttitle{Model-Based Recursive Partitioning in \proglang{R}}

\Keywords{parametric models, object-orientation, recursive partitioning}

\Abstract{  
  MOB is a generic algorithm for \underline{mo}del-\underline{b}ased recursive
  partitioning (Zeileis, Hothorn and Hornik 2008). Rather than fitting one
  global model to a dataset, it estimates local models on subsets of data that
  are ``learned'' by recursively partitioning. It proceeds in the following way:
  (1)~fit a parametric model to a data set, (2)~test for parameter instability
  over a set of partitioning variables, (3)~if there is some overall parameter
  instability, split the model with respect to the variable associated with the
  highest instability, (4)~repeat the procedure in each of the resulting subsamples. It
  is discussed how these steps of the conceptual algorithm are translated into
  computational tools in an object-oriented manner, allowing the user to plug in
  various types of parametric models. For representing the resulting trees, the
  \proglang{R} package \pkg{partykit} is employed and extended with generic
  infrastructure for recursive partitions where nodes are associated with
  statistical models. Compared to the previously available implementation
  in the \pkg{party} package, the new implementation supports more inference
  options, is easier to extend to new models, and provides more convenience
  features.
}

\Address{
  Achim Zeileis \\
  Department of Statistics \\
  Faculty of Economics and Statistics \\
  Universit\"at Innsbruck \\
  Universit\"atsstr.~15 \\
  6020 Innsbruck, Austria \\
  E-mail: \email{Achim.Zeileis@R-project.org} \\
  URL: \url{https://www.zeileis.org/} \\

  Torsten Hothorn\\
  Institut f\"ur Epidemiologie, Biostatistik und Pr\"avention \\
  Universit\"at Z\"urich \\
  Hirschengraben 84\\
  CH-8001 Z\"urich, Switzerland \\
  E-mail: \email{Torsten.Hothorn@R-project.org}\\
  URL: \url{https://user.math.uzh.ch/hothorn/}\\
}

\begin{document}

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\section{Overview}

To implement the model-based recursive partitioning (MOB) algorithm of 
\cite{Zeileis+Hothorn+Hornik:2008} in software, infrastructure for three
aspects is required: (1)~statistical ``\emph{models}'', (2)~recursive
``\emph{party}''tions, and (3)~``\emph{mobsters}'' carrying out the MOB
algorithm.

Along with \cite{Zeileis+Hothorn+Hornik:2008}, an implementation of all
three steps was provided in the \pkg{party} package \citep{pkg:party} for
the \proglang{R} system for statistical computing \citep{R}. This provided
one very flexible \code{mob()} function combining \pkg{party}'s \proglang{S}4
classes for representing trees with binary splits and the \proglang{S}4 model
wrapper functions from \pkg{modeltools} \citep{pkg:modeltools}. However, while
this supported many applications of interest, it was somewhat limited in several
directions: (1)~The \proglang{S}4 wrappers for the models were
somewhat cumbersome to set up. (2)~The tree infrastructure was originally
designed for \code{ctree()} and somewhat too narrowly focused on it.
(3)~Writing new ``mobster'' interfaces was not easy because of using
unexported \proglang{S}4 classes.

Hence, a leaner and more flexible interface (based on \proglang{S}3 classes)
is now provided in \pkg{partykit} \citep{pkg:partykit}: (1)~New models are much easier
to provide in a basic version and customization does not require setting up
an additional \proglang{S}4 class-and-methods layer anymore. (2)~The trees
are built on top of \pkg{partykit}'s flexible `\code{party}' objects,
inheriting many useful methods and providing new ones dealing with the
fitted models associated with the tree's nodes. (3)~New ``mobsters''
dedicated to specific models, e.g., \code{lmtree()} and \code{glmtree()}
for MOBs of (generalized) linear models, are readily provided.

The remainder of this vignette is organized as follows: Section~\ref{sec:algorithm}
very briefly reviews the original MOB algorithm of \cite{Zeileis+Hothorn+Hornik:2008}
and also highlights relevant subsequent work. Section~\ref{sec:implementation}
introduces the new \code{mob()} function in \pkg{partykit} in detail,
discussing how all steps of the MOB algorithm are implemented and which
options for customization are available. For illustration logistic-regression-based
recursive partitioning is applied to the Pima Indians diabetes data set from the
UCI machine learning repository \citep{mlbench2}. Section~\ref{sec:illustration}
and~\ref{sec:mobster} present further illustrative examples \citep[including replications from][]{Zeileis+Hothorn+Hornik:2008}
before Section~\ref{sec:conclusion} provides some concluding remarks.


\section{MOB: Model-based recursive partitioning} \label{sec:algorithm}

First, the theory underling the MOB (model-based recursive partitioning)
is briefly reviewed; a more detailed discussion is provided by \cite{Zeileis+Hothorn+Hornik:2008}.
To fix notation, consider a parametric model $\mathcal{M}(Y, \theta)$
with (possibly vector-valued) observations $Y$ and a
$k$-dimensional vector of parameters $\theta$. This model 
could be a (possibly multivariate) normal distribution for $Y$, 
a psychometric model for a matrix of responses $Y$, or some
kind of regression model when $Y = (y, x)$ can be split up into a dependent variable
$y$ and regressors $x$. An example for the latter could be a linear regression
model $y = x^\top \theta$ or a generalized linear model (GLM) or a survival 
regression.

Given $n$ observations $Y_i$ ($i = 1, \dots, n$) the model can be fitted
by minimizing some objective function $\sum_{i = 1}^n \Psi(Y_i, \theta)$, e.g., a residual sum of squares
or a negative log-likelihood leading to ordinary least squares (OLS) or maximum
likelihood (ML) estimation, respectively.

If a global model for all $n$ observations does not fit well and further
covariates $Z_1, \dots, Z_\ell$ are available, it might be possible to partition
the $n$ observations with respect to these variables and find a fitting local model
in each cell of the partition. The MOB algorithm tries to find
such a partition adaptively using a greedy forward search. The reasons
for looking at such local models might be different for different types of
models: First, the detection of interactions and nonlinearities in
regression relationships might be of interest just like in standard
classification and regression trees \citep[see e.g.,][]{Zeileis+Hothorn+Hornik:2008}.
Additionally, however, this approach allows to add explanatory variables to
models that otherwise do not have regressors or where the link between the
regressors and the parameters of the model is inclear \citep[this idea is
pursued by][]{Strobl+Wickelmaier+Zeileis:2011}. Finally, the model-based
tree can be employed as a thorough diagnostic test of the parameter stability
assumption (also termed measurement invariance in psychometrics). If the tree does
not split at all, parameter stability (or measurement invariance) cannot be rejected
while a tree with splits would indicate in which way the assumption is violated
\citep[][employ this idea in psychometric item response theory models]{Strobl+Kopf+Zeileis:2015}.

The basic idea is to grow a tee in which every node is associated with a model
of type $\mathcal{M}$. To assess whether splitting of the node is necessary a
fluctuation test for parameter instability is performed. If there is significant instability
with respect to any of the partitioning variables $Z_j$, the node is splitted
into $B$ locally optimal segments (the currenct version of the software has
$B = 2$ as the default and as the only option for numeric/ordered variables)
and then the procedure is repeated in each of the $B$ children.
If no more significant instabilities can be found, the recursion stops.
More precisely, the steps of the algorithm are
%
\begin{enumerate}
\item Fit the model once to all observations in the current node.
\item Assess whether the parameter estimates are stable with respect to
  every partitioning variable $Z_1, \dots, Z_\ell$. If there is some overall instability,
  select the variable $Z_j$ associated with the highest parameter instability, otherwise
  stop.
\item Compute the split point(s) that locally optimize the objective function $\Psi$.
\item Split the node into child nodes and repeat the procedure until some stopping criterion is met.
\end{enumerate}
%
This conceptual framework is extremely flexible and allows to adapt it to various
tasks by choosing particular models, tests, and methods in each of the steps:
%
\begin{enumerate}
\item \emph{Model estimation:} The original MOB introduction \citep{Zeileis+Hothorn+Hornik:2008}
  discussed regression models: OLS regression, GLMs, and survival regression. Subsequently,
  \cite{Gruen+Kosmidis+Zeileis:2012} have also adapted MOB to beta regression for limited
  response variables. Furthermore, MOB provides a generic way of adding covariates to models
  that otherwise have no regressors: this can either serve as a check whether the model is
  indeed independent from regressors or leads to local models for subsets. Both views are
  of interest when employing MOB to detect parameter instabilities in psychometric models
  for item responses such as the Bradley-Terry or the Rasch model
  \citep[see][respectively]{Strobl+Wickelmaier+Zeileis:2011,Strobl+Kopf+Zeileis:2015}.
\item \emph{Parameter instability tests:} To assess the stability of all model parameters
  across a certain partitioning variable, the general class of score-based fluctuation tests proposed by
  \cite{Zeileis+Hornik:2007} is employed. Based on the empirical score function observations
  (i.e., empirical estimating functions or contributions to the gradient), ordered with
  respect to the partitioning variable, the fluctuation or instability in the model's
  parameters can be tested. From this general framework the Andrews' sup\textit{LM} test
  is used for assessing numerical partitioning variables and a $\chi^2$ test for
  categorical partitioning variables (see \citealp{Zeileis:2005} and \citealp{Merkle+Zeileis:2013}
  for unifying views emphasizing regression and psychometric models, respectively).
  Furthermore, the test statistics for ordinal partitioning variables suggested by
  \cite{Merkle+Fan+Zeileis:2014} have been added as further options (but are not used
  by default as the simulation of $p$-values is computationally demanding).
\item \emph{Partitioning:} As the objective function $\Psi$ is additive, it is easy
  to compute a single optimal split point (or cut point or break point). For each conceivable
  split, the model is estimated on the two resulting subsets and the resulting objective
  functions are summed. The split that optimizes this segmented objective function
  is then selected as the optimal split. For optimally splitting the data into $B > 2$
  segments, the same idea can be used and a full grid search can be avoided by employing
  a dynamic programming algorithms \citep{Hawkins:2001,Bai+Perron:2003} but at the moment
  the latter is not implemented in the software. Optionally, however, categorical partitioning
  variables can be split into all of their categories (i.e., in that case $B$ is set to
  the number of levels without searching for optimal groupings).
\item \emph{Pruning:} For determining the optimal size of the tree, one can either use
  a pre-pruning or post-pruning strategy. For the former, the algorithm stops when
  no significant parameter instabilities are detected in the current node (or when the
  node becomes too small). For the latter, one would first grow a large tree (subject only
  to a minimal node size requirement) and then prune back splits that did not improve
  the model, e.g., judging by information criteria such as AIC or BIC \citep[see e.g.,][]{Su+Wang+Fan:2004}.
  Currently, pre-pruning is used by default (via Bonferroni-corrected $p$-values
  from the score-based fluctuation tests) but AIC/BIC-based post-pruning is optionally
  available (especially for large data sets where traditional significance levels
  are not useful).
\end{enumerate}
%
In the following it is discussed how most of the options above are embedded
in a common computational framework using \proglang{R}'s facilities for model estimation
and object orientation.

\section[A new implementation in R]{A new implementation in \proglang{R}} \label{sec:implementation}

This section introduces a new implementation of the general model-based recursive
partitioning (MOB) algorithm in \proglang{R}. Along with \cite{Zeileis+Hothorn+Hornik:2008},
a function \code{mob()} had been introduced to the \pkg{party} package
\citep{pkg:party}
which continues to work but it turned out to be somewhat unflexible for certain
applications/extensions. Hence, the \pkg{partykit} package \citep{pkg:partykit} provides a completely rewritten
(and not backward compatible) implementation of a new \code{mob()} function along with
convenience interfaces \code{lmtree()} and \code{glmtree()} for fitting linear model
and generalized linear model trees, respectively.
The function \code{mob()} itself is intended to be the workhorse function
that can also be employed to quickly explore new models -- whereas \code{lmtree()}
and \code{glmtree()} will be the typical user interfaces facilitating applications.

The new \code{mob()} function has the following arguments:
\begin{Code}
  mob(formula, data, subset, na.action, weights, offset,
    fit, control = mob_control(), ...)
\end{Code}
All arguments in the first line are standard for modeling functions in \proglang{R}
with a \code{formula} interface. They are employed by \code{mob()} to do some
data preprocessing (described in detail in Section~\ref{sec:formula}) before
the \code{fit} function is used for parameter estimation on the preprocessed
data. The \code{fit} function has to be set up in a certain way (described in
detail in Section~\ref{sec:fit}) so that \code{mob()} can extract all information
that is needed in the MOB algorithm for parameter instability tests (see Section~\ref{sec:sctest})
and partitioning/splitting (see Section~\ref{sec:split}), i.e., the estimated parameters, the associated
objective function, and the score function contributions. A list of \code{control}
options can be set up by the \code{mob_control()} function, including options
for pruning (see Section~\ref{sec:prune}). Additional arguments \code{...}
are passed on to the \code{fit} function.

The result is an object of class `\code{modelparty}' inheriting from `\code{party}'.
The \code{info} element of the overall `\code{party}' and the individual
`\code{node}'s contain various informations about the models. Details are provided
in Section~\ref{sec:object}.

Finally, a wide range of standard (and some extra) methods are available for
working with `\code{modelparty}' objects, e.g., for extracting information about the
models, for visualization, computing predictions, etc. The standard set of methods
is introduced in Section~\ref{sec:methods}. However, as will be discussed there,
it may take some effort by the user to efficiently compute certain pieces of
information. Hence, convenience interfaces such as \code{lmtree()} or \code{glmtree()}
can alleviate these obstacles considerably, as illustrated in Section~\ref{sec:interface}.


\subsection{Formula processing and data preparation} \label{sec:formula}

The formula processing within \code{mob()} is essentially done in ``the usual way'',
i.e., there is a \code{formula} and \code{data} and optionally further arguments such
as \code{subset}, \code{na.action}, \code{weights}, and \code{offset}. These
are processed into a \code{model.frame} from which the response and the covariates
can be extracted and passed on to the actual \code{fit} function.

As there are possibly three groups of variables (response, regressors,
and partitioning variables), the \pkg{Formula} package \citep{Formula} is
employed for processing these three parts. Thus, the formula can be of type
\verb:y ~ x1 + ... + xk | z1 + ... + zl: where the variables on the
left of the \code{|} specify the data $Y$ and the variables on the right specify the
partitioning variables $Z_j$. As pointed out above, the $Y$ can often be split
up into response (\code{y} in the example above) and regressors (\code{x1}, \dots,
\code{xk} in the example above). If there are no regressors and just constant
fits are employed, then the formula can be specified as
\verb:y ~ 1 | z1 + ... + zl:.

So far, this formula representation is really just a specification of groups
of variables and does not imply anything about the type of model that is to be
fitted to the data in the nodes of the tree. The \code{mob()} function does not
know anything about the type of model and just passes (subsets of) the \code{y}
and \code{x} variables on to the \code{fit} function. Only the partitioning variables
\code{z} are used internally for the parameter instability tests (see Section~\ref{sec:sctest}).

As different \code{fit} functions will require the data in different formats,
\code{mob_control()} allows to set the \code{ytype} and \code{xtype}. The default
is to assume that \code{y} is just a single column of the model frame that is
extracted as a \code{ytype = "vector"}. Alternatively, it can be a \code{"data.frame"}
of all response variables or a \code{"matrix"} set up via \code{model.matrix()}.
The options \code{"data.frame"} and \code{"matrix"} are also available for \code{xtype}
with the latter being the default. Note that if \code{"matrix"} is used, then transformations
(e.g., logs, square roots etc.) and dummy/interaction codings are computed and turned
into columns of a numeric matrix while for \code{"data.frame"} the original variables are preserved.

By specifying the \code{ytype} and \code{xtype}, \code{mob()} is also provided
with the information on how to correctly subset \code{y} and \code{x} when recursively
partitioning data. In each step, only the subset of \code{y} and \code{x} pertaining to
the current node of the tree is passed on to the \code{fit} function. Similarly,
subsets of \code{weights} and \code{offset} are passed on (if specified).

\subsubsection*{Illustration}

For illustration, we employ the popular benchmark data set on diabetes among
Pima Indian women that is provided by the UCI machine learning repository \citep{mlbench2} and 
available in the \pkg{mlbench} package \citep{pkg:mlbench}:
%
<<PimaIndiansDiabetes>>=
data("PimaIndiansDiabetes", package = "mlbench")
@
%
Following \cite{Zeileis+Hothorn+Hornik:2008} we want to fit a model for \code{diabetes}
employing the plasma glucose concentration \code{glucose} as a regressor. As the influence
of the remaining variables on \code{diabetes} is less clear, their relationship should be
learned by recursive partitioning. Thus, we employ the following formula:
%
<<PimIndiansDiabetes-formula>>=
pid_formula <- diabetes ~ glucose | pregnant + pressure + triceps +
  insulin + mass + pedigree + age
@
%
Before passing this to \code{mob()}, a \code{fit} function is needed and a logistic regression
function is set up in the following section.

\subsection{Model fitting and parameter estimation} \label{sec:fit}

The \code{mob()} function itself does not actually carry out any parameter
estimation by itself, but assumes that one of the many \proglang{R} functions
available for model estimation is supplied. However, to be able to carry out the steps
of the MOB algorithm, \code{mob()} needs to able to extract certain pieces
of information: especially the estimated parameters, the corresponding
objective function, and associated score function contributions. Also,
the interface of the fitting function clearly needs to be standardized so
that \code{mob()} knows how to invoke the model estimation.

Currently, two possible interfaces for the \code{fit} function can be
employed:
%
\begin{enumerate}

\item The \code{fit} function can take the following inputs
\begin{Code}
  fit(y, x = NULL, start = NULL, weights = NULL, offset = NULL, ...,
    estfun = FALSE, object = FALSE)
\end{Code}
where \code{y}, \code{x}, \code{weights}, \code{offset} are (the subset of)
the preprocessed data. In \code{start} starting values for the parameter
estimates may be supplied and \code{...} is passed on from the \code{mob()}
function. The \code{fit} function then has to return an output list with
the following elements:
\begin{itemize}
  \item \code{coefficients}: Estimated parameters $\hat \theta$.
  \item \code{objfun}: Value of the minimized objective function $\sum_i \Psi(y_i, x_, \hat \theta)$.
  \item \code{estfun}: Empirical estimating functions (or score function contributions)
    $\Psi'(y_i, x_i, \hat \theta)$. Only needed if \code{estfun = TRUE}, otherwise optionally \code{NULL}.
  \item \code{object}: A model object for which further methods could
    be available (e.g., \code{predict()}, or \code{fitted()}, etc.).
    Only needed if \code{object = TRUE}, otherwise optionally \code{NULL}.
\end{itemize}
By making \code{estfun} and \code{object} optional, the fitting function
might be able to save computation time by only optimizing the objective
function but avoiding further computations (such as setting up covariance
matrix, residuals, diagnostics, etc.).

\item The \code{fit} function can also of a simpler interface with only the
following inputs:
\begin{Code}
  fit(y, x = NULL, start = NULL, weights = NULL, offset = NULL, ...)
\end{Code}
The meaning of all arguments is the same as above. However, in this case
\code{fit} needs to return a classed model object for which methods to
\code{coef()}, \code{logLik()}, and \code{estfun()} \citep[see][and the \pkg{sandwich} package]{sandwich}
are available for extracting the parameter estimates, the maximized log-likelihood,
and associated empirical estimating functions (or score contributions), respectively.
Internally, a function of type (1) is set up by \code{mob()} in case a function
of type (2) is supplied. However, as pointed out above, a function of type (1)
might be useful to save computation time.

\end{enumerate}
%
In either case the \code{fit} function can, of course, choose to ignore any
arguments that are not applicable, e.g., if the are no regressors \code{x} in
the model or if starting values or not supported.

The \code{fit} function of type (2) is typically convenient to quickly try out
a new type of model in recursive partitioning.  However, when writing a new
\code{mob()} interface such as \code{lmtree()} or \code{glmtree()},
it will typically be better to use type (1). Similarly, employing supporting
starting values in \code{fit} is then recommended to save computation
time.

\subsubsection*{Illustration}

For recursively partitioning the \code{diabetes ~ glucose} relationship (as already
set up in the previous section), we employ a logistic regression model. An
interface of type (2) can be easily set up:
%
<<logit>>=
logit <- function(y, x, start = NULL, weights = NULL, offset = NULL, ...) {
  glm(y ~ 0 + x, family = binomial, start = start, ...)
}
@
%
Thus, \code{y}, \code{x}, and the starting values are passed on to \code{glm()}
which returns an object of class `\code{glm}' for which all required methods
(\code{coef()}, \code{logLik()}, and \code{estfun()}) are available.

Using this \code{fit} function and the \code{formula} already set up above
the MOB algorithm can be easily applied to the \code{PimaIndiansDiabetes}
data:
%
<<PimaIndiansDiabetes-mob>>=
pid_tree <- mob(pid_formula, data = PimaIndiansDiabetes, fit = logit)
@
%
The result is a logistic regression tree with three terminal nodes that
can be easily visualized via \code{plot(pid_tree)} (see Figure~\ref{fig:pid_tree})
and printed:
<<PimaIndiansDiabetes-print>>=
pid_tree
@
%
The tree finds three groups of Pima Indian women:
\begin{itemize}
  \item[\#2] Women with low body mass index that have on average a low risk of
    diabetes, however this increases clearly with glucose level.
  \item[\#4] Women with average and high body mass index, younger than 30 years,
    that have a higher avarage risk that also increases with glucose level.
  \item[\#5] Women with average and high body mass index, older than 30 years,
    that have a high avarage risk that increases only slowly with glucose level.
\end{itemize}


Note that the example above is used for illustration here and \code{glmtree()} is
recommended over using \code{mob()} plus manually setting up a \code{logit()} function.
The same tree structure can be found via:
%
<<PimaIndiansDiabetes-glmtree>>=
pid_tree2 <- glmtree(diabetes ~ glucose | pregnant +
  pressure + triceps + insulin + mass + pedigree + age,
  data = PimaIndiansDiabetes, family = binomial)
@
%
However, \code{glmtree()} is slightly faster as it avoids many unnecessary computations,
see Section~\ref{sec:interface} for further details.

\begin{figure}[p!]
\centering
\setkeys{Gin}{width=0.8\textwidth}
<<PimaIndiansDiabetes-plot, echo=FALSE, results='hide', fig.height=5, fig.width=7>>=
plot(pid_tree)
@
\caption{\label{fig:pid_tree} Logistic-regression-based tree for the Pima Indians diabetes
data. The plots in the leaves report the estimated regression coefficients.}

\setkeys{Gin}{width=\textwidth}
<<PimaIndiansDiabetes-plot2,echo=FALSE,results='hide',fig.height=6,fig.width=10>>=
plot(pid_tree2, tp_args = list(ylines = 1, margins = c(1.5, 1.5, 1.5, 2.5)))
@
\caption{\label{fig:pid_tree2} Logistic-regression-based tree for the Pima Indians
diabetes data. The plots in the leaves give spinograms for \code{diabetes} versus 
\code{glucose}.}
\end{figure}

Here, we only point out that \code{plot(pid_tree2)} produces a nicer visualization
(see Figure~\ref{fig:pid_tree2}). As $y$ is \code{diabetes}, a binary variable, and $x$ is 
\code{glucose}, a numeric variable, a spinogram is chosen automatically for visualization
(using the \pkg{vcd} package, \citealp{vcd}).
The x-axis breaks in the spinogram are the five-point summary of \code{glucose} on the full data set. The
fitted lines are the mean predicted probabilities in each group.



\subsection{Testing for parameter instability} \label{sec:sctest}

In each node of the tree, first the parametric model is fitted to all observations
in that node (see Section~\ref{sec:fit}). Subsequently it is of interest
to find out whether the model parameters are stable over each particular ordering implied by
the partitioning variables $Z_j$ or whether splitting the sample with respect
to one of the $Z_j$ might capture instabilities in the parameters and thus improve the fit.
The tests used in this step belong to the class of generalized M-fluctuation
tests \citep{Zeileis:2005,Zeileis+Hornik:2007}. Depending on the class of each
of the partitioning variables in \code{z} different types of tests are chosen by \code{mob()}:
\begin{enumerate}

\item For numeric partitioning variables (of class `\code{numeric}' or `\code{integer}')
the sup\textit{LM}~statistic is used which is the maximum over all single-split \textit{LM} statistics.
Associated $p$-values can be obtained from a table of critical values \citep[based on][]{Hansen:1997}
stored within the package.

If there are ties in the partitioning variable, the sequence of \textit{LM} statistics (and hence
their maximum) is not unique and hence the results by default may depend to some degree on the
ordering of the observations. To explore this, the option \code{breakties = TRUE} can be
set in \code{mob_control()} which breaks ties randomly. If there are are only few ties,
the influence is often negligible. If there are many ties (say only a dozen unique values of
the partitioning variable), then the variable may be better treated as an ordered factor (see below).

\item For categorical partitioning variables (of class `\code{factor}'), a $\chi^2$~statistic is
employed which captures the fluctuation within each of the categories of the partitioning variable.
This is also an \textit{LM}-type test and is asymptotically equivalent to the corresponding
likelihood ratio test. However, it is somewhat cheaper to compute the \textit{LM} statistic
because the model just has to be fitted once in the current node and not separately for each
category of each possible partitioning variable. See also \cite{Merkle+Fan+Zeileis:2014} for
a more detailed discussion.

\item For ordinal partitioning variables (of class `\code{ordered}' inheriting from `\code{factor}')
the same $\chi^2$ as for the unordered categorical variables is used by default
\citep[as suggested by][]{Zeileis+Hothorn+Hornik:2008}. Although this test is
consistent for any parameter instabilities across ordered variables, it does
not exploit the ordering information.

Recently, \cite{Merkle+Fan+Zeileis:2014}
proposed an adapted max\textit{LM} test for ordered variables and, alternatively, a
weighted double maximum test. Both are optionally availble in the new \code{mob()}
implementation by setting \code{ordinal = "L2"} or \code{ordinal = "max"}
in \code{mob_control()}, respectively. Unfortunately, computing $p$-values from
both tests is more costly than for the default \code{ordinal = "chisq"}. For
\code{"L2"} suitable tables of critical values have to be simulated on the
fly using \code{ordL2BB()} from the \pkg{strucchange} package \citep{strucchange}.
For \code{"max"} a multivariate normal probability has to be computed using
the \pkg{mvtnorm} package \citep{pkg:mvtnorm}.

\end{enumerate}
All of the parameter instability tests above can be computed in an object-oriented manner
as the ``\code{estfun}'' has to be available for the fitted model object. (Either
by computing it in the \code{fit} function directly or by providing a \code{estfun()}
extractor, see Section~\ref{sec:fit}.)


All tests also require an estimate of the
corresponding variance-covariance matrix of the estimating functions. The default
is to compute this using an outer-product-of-gradients (OPG) estimator. Alternatively,
the corrsponding information matrix or sandwich matrix can be used if: (a)~the estimating functions
are actually maximum likelihood scores, and (b)~a \code{vcov()} method (based on
an estimate of the information) is provided for the fitted model objects. The corresponding
option in \code{mob_control()} is to set \code{vcov = "info"} or \code{vcov = "sandwich"}
rather than \code{vcov = "opg"} (the default).

For each of the $j = 1, \dots, \ell$ partitioning variables in \code{z} the test selected
in the control options is employed and the corresponding $p$-value $p_j$ is computed.
To adjust for multiple testing, the $p$ values can be Bonferroni adjusted
(which is the default). To determine whether there is some overall instability, it is checked whether
the minial $p$-value $p_{j^*} = \min_{j = 1, \dots, \ell} p_j$ falls below a
pre-specified significance level $\alpha$ (by default $\alpha = 0.05$) or not.
If there is significant instability, the variable $Z_{j^*}$
associated with the minimal $p$-value is used for splitting the node.

\subsubsection*{Illustration}

In the logistic-regression-based MOB \code{pid_tree} computed above, the parameter instability
tests can be extracted using the \code{sctest()} function from the \pkg{strucchange}
package (for \underline{s}tructural \underline{c}hange \underline{test}). In the first
node, the test statistics and Bonferroni-corrected $p$-values are:
%
<<PimaIndiansDiabetes-sctest1>>=
library("strucchange")
sctest(pid_tree, node = 1)
@
%
Thus, the body \code{mass} index has the lowest $p$-value and is highly significant
and hence used for splitting the data. In the second node, no further significant
parameter instabilities can be detected and hence partitioning stops in that branch.
%
<<PimaIndiansDiabetes-sctest2>>=
sctest(pid_tree, node = 2)
@
%
In the third node, however, there is still significant instability associated with the
\code{age} variable and hence partitioning continues.
%
<<PimaIndiansDiabetes-sctest3>>=
sctest(pid_tree, node = 3)
@
%
Because no further instabilities can be found in the fourth and fifth node, the
recursive partitioning stops there.


\subsection{Splitting} \label{sec:split}

In this step, the observations in the current node are split with respect
to the chosen partitioning variable $Z_{j^*}$ into $B$ child nodes. As pointed out above,
the \code{mob()} function currently only supports binary splits, i.e., $B = 2$.
For deterimining the split point, an exhaustive search procedure is adopted:
For each conceivable split point in $Z_{j^*}$, the two subset models are fit and the split
associated with the minimal value of the objective function $\Psi$ is chosen.

Computationally, this means that the \code{fit} function is applied to the subsets
of \code{y} and \code{x} for each possibly binary split. The ``\code{objfun}'' values
are simply summed up (because the objective function is assumed to be additive) and
its minimum across splits is determined. In a search over a numeric partitioning variable,
this means that typically a lot of subset models have to be fitted and often these will not
vary a lot from one split to the next. Hence, the parameter estimates ``\code{coefficients}''
from the previous split are employed as \code{start} values for estimating the coefficients
associated with the next split. Thus, if the \code{fit} function makes use of these
starting values, the model fitting can often be speeded up.

\subsubsection*{Illustration}

For the Pima Indians diabetes data, the split points found for \code{pid_tree}
are displayed both in the print output and the visualization (see Figure~\ref{fig:pid_tree}
and~\ref{fig:pid_tree2}).

\subsection{Pruning} \label{sec:prune}

By default, the size of \code{mob()} trees is determined only by the significance tests,
i.e., when there is no more significant parameter instability (by default at level $\alpha = 0.05$)
the tree stops growing. Additional stopping criteria are only the minimal node size (\code{minsize})
and the maximum tree depth (\code{maxdepth}, by default infinity).

However, for very large sample size traditional significance levels are typically not useful
because even tiny parameter instabilities can be detected. To avoid overfitting in such a situation,
one would either have to use much smaller significance levels or add some form of post-pruning
to reduce the size of the tree afterwards. Similarly, one could wish to first grow a very large
tree (using a large $\alpha$ level) and then prune it afterwards, e.g., using some information
criterion like AIC or BIC.

To accomodate such post-pruning strategies, \code{mob_control()} has an argument \code{prune} that
can be a \code{function(objfun, df, nobs)} that either returns \code{TRUE} if a node should be
pruned or \code{FALSE} if not. The arguments supplied are a vector of objective function values in
the current node and the sum of all child nodes, a vector of corresponding degrees of freedom,
and the number of observations in the current node and in total. For example if the objective
function used is the negative log-likelihood, then for BIC-based pruning the \code{prune} function
is: \code{(2 * objfun[1] + log(nobs[1]) * df[1]) < (2 * objfun[2] + log(nobs[2]) * df[2])}. As the
negative log-likelihood is the default objective function when using the ``simple'' \code{fit}
interface, \code{prune} can also be set to \code{"AIC"} or \code{"BIC"} and then suitable
functions will be set up internally. Note, however, that for other objective functions this
strategy is not appropriate and the functions would have to be defined differently (which
\code{lmtree()} does for example).

In the literature, there is no clear consensus as to how many degrees of freedom should be
assigned to the selection of a split point. Hence, \code{mob_control()} allows to set
\code{dfsplit} which by default is \code{dfsplit = TRUE} and then \code{as.integer(dfsplit)}
(i.e., 1 by default) degrees of freedom per split are used. This can be modified to
\code{dfsplit = FALSE} (or equivalently \code{dfsplit = 0}) or \code{dfsplit = 3} etc.\
for lower or higher penalization of additional splits.

\subsubsection*{Illustration}

With $n = \Sexpr{nrow(PimaIndiansDiabetes)}$ observations, the sample size is quite
reasonable for using the classical significance level of $\alpha = 0.05$ which is also reflected
by the moderate tree size with three terminal nodes. However, if we wished to explore further
splits, a conceivable strategy could be the following:
%
<<PimaIndiansDiabetes-prune, eval=FALSE>>=
pid_tree3 <- mob(pid_formula, data = PimaIndiansDiabetes,
  fit = logit, control = mob_control(verbose = TRUE,
    minsize = 50, maxdepth = 4, alpha = 0.9, prune = "BIC"))
@
This first grows a large tree until the nodes become too small (minimum node size: 50~observations)
or the tree becomes too deep (maximum depth 4~levels) or the significance levels come very close
to one (larger than $\alpha = 0.9$). Here, this large tree has eleven nodes which are subsequently
pruned based on whether or not they improve the BIC of the model. For this data set, the resulting
BIC-pruned tree is in fact identical to the pre-pruned \code{pid_tree} considered above.

\subsection[Fitted `party' objects]{Fitted `\texttt{party}' objects} \label{sec:object}

The result of \code{mob()} is an object of class `\code{modelparty}' inheriting from `\code{party}'.
See the other vignettes in the \pkg{partykit} package \citep{pkg:partykit} for more details
of the general `\code{party}' class. Here, we just point out that the main difference
between a `\code{modelparty}' and a plain `\code{party}' is that additional information
about the data and the associated models is stored in the \code{info} elements:
both of the overall `\code{party}' and the individual `\code{node}'s. The details
are exemplified below.

\subsubsection*{Illustration}

In the \code{info} of the overall `\code{party}' the following information is stored for
\code{pid_tree}:
%
<<PimaIndiansDiabetes-info>>=
names(pid_tree$info)
@
%
The \code{call} contains the \code{mob()} call. The \code{formula} and \code{Formula}
are virtually the same but are simply stored as plain `\code{formula}' and extended
`\code{Formula}' \citep{Formula} objects, respectively. The \code{terms} contain separate
lists of terms for the \code{response} (and regressor) and the \code{partitioning} variables.
The \code{fit}, \code{control} and \code{dots} are the arguments that were provided
to \code{mob()} and \code{nreg} is the number of regressor variables.

Furthermore, each \code{node} of the tree contains the following information:
%
<<PimaIndiansDiabetes-info-tree>>=
names(pid_tree$node$info)
@
%
The \code{coefficients}, \code{objfun}, and \code{object} are the results of the \code{fit}
function for that node. In \code{nobs} and \code{p.value} the number of observations
and the minimal $p$-value are provided, respectively. Finally, \code{test} contains
the parameter instability test results. Note that the \code{object} element can also
be suppressed through \code{mob_control()} to save memory space.


\subsection{Methods} \label{sec:methods}

There is a wide range of standard methods available for objects of class `\code{modelparty}'.
The standard \code{print()}, \code{plot()}, and \code{predict()} build on the 
corresponding methods for `\code{party}' objects but provide some more special options.
Furthermore, methods are provided that are typically available for models with formula interfaces:
\code{formula()} (optionally one can set \code{extended = TRUE} to get the `\code{Formula}'), \code{getCall()},
\code{model.frame()}, \code{weights()}. Finally, there is a standard set of methods
for statistical model objects: \code{coef()}, \code{residuals()}, \code{logLik()} (optionally
setting \code{dfsplit = FALSE} suppresses counting the splits in the degrees of freedom,
see Section~\ref{sec:prune}),
\code{deviance()}, \code{fitted()}, and \code{summary()}.

Some of these methods rely on reusing the corresponding methods for the individual model
objects in the terminal nodes. Functions such as \code{coef()}, \code{print()}, \code{summary()}
also take a \code{node} argument that can specify the node IDs to be queried.

Two methods have non-standard arguments to allow for additional flexibility when dealing
with model trees. Typically, ``normal'' users do not have to use these arguments directly
but they are very flexible and facilitate writing convenience interfaces such as \code{glmtree()}
(see Section~\ref{sec:interface}).
\begin{itemize}
\item The \code{predict()} method has the following arguments:
\code{predict(object, newdata = NULL, type = "node", ...)}.
The argument \code{type} can either be a function or a character string.
More precisely, if \code{type} is a function it should be a
\code{function (object, newdata = NULL, ...)} that returns a vector or
matrix of predictions from a fitted model \code{object} either with or
without \code{newdata}. If \code{type} is a character string, such a
function is set up internally as
\code{predict(object, newdata = newdata, type = type, ...)}, i.e.,
it relies on a suitable \code{predict()} method being available for
the fitted models associated with the `\code{party}' object.

\item The \code{plot()} method has the following arguments:
\code{plot(x, terminal_panel = NULL, FUN = NULL)}.
This simply calls the \code{plot()} method for `\code{party}' objects with a
custom panel function for the terminal panels. By default, \code{node_terminal} is used
to include some short text in each terminal node. This text can be set up by \code{FUN}
with the default being the number of observations and estimated parameters. However,
more elaborate terminal panel functions can be written, as done for the convenience
interfaces.
\end{itemize}

Finally, two \proglang{S}3-style functions are provided without the corresponding
generics (as these reside in packages that \pkg{partykit} does not depend on).
The \code{sctest.modelparty} can be used in combination with the \code{sctest()}
generic from \pkg{strucchange} as illustrated in Section~\ref{sec:sctest}. The
\code{refit.modelparty} function extracts (or refits if necessary) the fitted
model objects in the specified nodes (by default from all nodes). 


\subsubsection*{Illustration}

The \code{plot()} and \code{print()} methods have already been illustrated for the \code{pid_tree}
above. However, here we add that the \code{print()} method can also be used to show
more detailed information about particular nodes instead of showing the full tree:
%
<<PimaIndiansDiabetes-print3>>=
print(pid_tree, node = 3)
@
%
Information about the model and coefficients can for example be extracted by:
%
<<PimaIndiansDiabetes-coef>>=
coef(pid_tree)
coef(pid_tree, node = 1)
## IGNORE_RDIFF_BEGIN
summary(pid_tree, node = 1)
## IGNORE_RDIFF_END
@
%
As the coefficients pertain to a logistic regression, they can be easily
interpreted as odds ratios by taking the \code{exp()}:
%
<<>>=
exp(coef(pid_tree)[,2])
@  
%
<<echo=FALSE>>=
risk <- round(100 * (exp(coef(pid_tree)[,2])-1), digits = 1)
@
%
i.e., the odds increase by \Sexpr{risk[1]}\%, \Sexpr{risk[2]}\% and \Sexpr{risk[3]}\%
with respect to glucose in the three terminal nodes.

Log-likelihoods and information criteria are available (which by default also
penalize the estimation of splits):
<<PimaIndiansDiabetes-logLik>>=
logLik(pid_tree)
AIC(pid_tree)
BIC(pid_tree)
@
%
Mean squared residuals (or deviances) can be extracted in different ways:
<<PimaIndiansDiabetes-deviance>>=
mean(residuals(pid_tree)^2)
deviance(pid_tree)/sum(weights(pid_tree))
deviance(pid_tree)/nobs(pid_tree)
@
%
Predicted nodes can also be easily obtained:
%
<<PimaIndiansDiabetes-predict>>=
pid <- head(PimaIndiansDiabetes)
predict(pid_tree, newdata = pid, type = "node")
@
%
More predictions, e.g., predicted probabilities within the nodes, can also
be obtained but require some extra coding if only \code{mob()} is used.
However, with the \code{glmtree()} interface this is very easy as shown
below.

Finally, several methods for `\code{party}' objects are, of course, also
available for `\code{modelparty}' objects, e.g., querying width and depth of
the tree:
%
<<PimaIndiansDiabetes-width>>=
width(pid_tree)
depth(pid_tree)
@
%
Also subtrees can be easily extracted:
%
<<PimaIndiansDiabetes-subset>>=
pid_tree[3]
@
%
The subtree is again a completely valid `\code{modelparty}' and hence it could
also be visualized via \code{plot(pid_tree[3])} etc.


\subsection{Extensions and convenience interfaces} \label{sec:interface}

As illustrated above, dealing with MOBs can be carried out in a very generic
and object-oriented way. Almost all information required for dealing with
recursively partitioned models can be encapsulated in the \code{fit()} function
and \code{mob()} does not require more information on what type of model is
actually used.

However, for certain tasks more detailed information about the type of model
used or the type of data it can be fitted to can (and should) be exploited.
Notable examples for this are visualizations of the data along with the fitted
model or model-based predictions in the leaves of the tree. To conveniently
accomodate such specialized functionality, the \pkg{partykit} provides two
convenience interfaces \code{lmtree()} and \code{glmtree()} and encourages
other packages to do the same (e.g., \code{raschtree()} and \code{bttree()}
in \pkg{psychotree}). Furthermore, such a convenience interface can avoid
duplicated formula processing in both \code{mob()} plus its \code{fit}
function.

Specifically, \code{lmtree()} and \code{glmtree()} interface \code{lm.fit()},
\code{lm.wfit()}, and \code{glm.fit()}, respectively, and then provide some
extra computations to return valid fitted `\code{lm}' and `\code{glm}' objects
in the nodes of the tree. The resulting `\code{modelparty}' object gains an
additional class `\code{lmtree}'/`\code{glmtree}' to dispatch to its enhanced
\code{plot()} and \code{predict()} methods.

\subsubsection*{Illustration}

The \code{pid_tree2} object was already created above with \code{glmtree()}
(instead of \code{mob()} as for \code{pid_tree}) to illustrate the enhanced
plotting capabilities in Figure~\ref{fig:pid_tree2}. Here, the enhanced
\code{predict()} method is used to obtain predicted means (i.e., probabilities)
and associated linear predictors (on the logit scale) in addition to the
previously available predicted nods IDs.
%
<<>>=
predict(pid_tree2, newdata = pid, type = "node")
predict(pid_tree2, newdata = pid, type = "response")
predict(pid_tree2, newdata = pid, type = "link")
@


\section{Illustrations}
\label{sec:illustration}

In the remainder of the vignette, further empirical illustrations of the MOB method are
provided, including replications of the results from \cite{Zeileis+Hothorn+Hornik:2008}:
\begin{enumerate}
  \item An investigation of the price elasticity of the demand for economics
    journals across covariates describing the type of journal (e.g., its
    price, its age, and whether it is published by a society, etc.)
  \item Prediction of house prices in the well-known Boston Housing data
    set, also taken from the UCI machine learning repository.
  \item Explore how teaching ratings and beauty scores of professors are
    associated and how this association changes across further explanatory
    variables such as age, gender, and native speaker status of the professors.
  \item Assessment of differences in the preferential treatment of women/children
    (``women and children first'') across subgroups of passengers on board of the
    ill-fated maiden voyage of the RMS Titanic.
  \item Modeling of breast cancer survival by capturing heterogeneity
    in certain (treatment) effects across patients.
  \item Modeling of paired comparisons of topmodel candidates
    by capturing heterogeneity in their attractiveness scores across
    participants in a survey.
\end{enumerate}
More details about several of the underlying data sets, previous studies exploring
the data, and the results based on MOB can be found in 
\cite{Zeileis+Hothorn+Hornik:2008}.

Here, we focus on using the \pkg{partykit} package to replicate the analysis
and explore the resulting trees. The first three illustrations employ the
\code{lmtree()} convenience function, the fourth is based on logistic regression
using \code{glmtree()}, and the fifth uses \code{survreg()}
from \pkg{survival} \citep{pkg:survival} in combination with \code{mob()} directly.
The sixth and last illustration is deferred to a separate section and shows
in detail how to set up new ``mobster'' functionality from scratch.

\subsection{Demand for economic journals}

The price elasticity of the demand for 180~economic journals is assessed by
an OLS regression in log-log form: The dependent variable is the logarithm of the number
of US library subscriptions and the regressor is the logarithm of price
per citation. The data are provided by the \pkg{AER} package \citep{AER}
and can be loaded and transformed via:
%
<<Journals-data>>=
data("Journals", package = "AER")
Journals <- transform(Journals,
  age = 2000 - foundingyear,
  chars = charpp * pages)
@
%
Subsequently, the stability of the price elasticity across the remaining
variables can be assessed using MOB. Below, \code{lmtree()} is used with the
following partitioning variables: raw price and citations, age of the journal,
number of characters, and whether the journal is published by a scientific
society or not. A minimal segment size of 10~observations is employed and
by setting \code{verbose = TRUE} detailed progress information about the
recursive partitioning is displayed while growing the tree:
%
<<Journals-tree>>=
j_tree <- lmtree(log(subs) ~ log(price/citations) | price + citations +
  age + chars + society, data = Journals, minsize = 10, verbose = TRUE)
@
%
\begin{figure}[t!]
\centering
\setkeys{Gin}{width=0.75\textwidth}
<<Journals-plot, echo=FALSE, results='hide', fig.height=5.5, fig.width=7>>=
plot(j_tree)
@
\caption{\label{fig:Journals} Linear-regression-based tree for the journals
data. The plots in the leaves show linear regressions of log(subscriptions) by
log(price/citation).}
\end{figure}
%
The resulting tree just has one split and two terminal nodes for young journals
(with a somewhat larger price elasticity) and old journals (with an even lower
price elasticity), respectively. Figure~\ref{fig:Journals} can be obtained
by \code{plot(j_tree)} and the corresponding printed representation is
shown below.
%
<<Journals-print>>=
j_tree
@
%
Finally, various quantities of interest such as the coefficients, standard
errors and test statistics, and the associated parameter instability tests
could be extracted by the following code. The corresponding output is suppressed here.
%
<<Journals-methods, results='hide'>>=
coef(j_tree, node = 1:3)
summary(j_tree, node = 1:3)
sctest(j_tree, node = 1:3)
@

\subsection{Boston housing data}

The Boston housing data are a popular and well-investigated empirical basis for
illustrating nonlinear regression methods both in machine learning and statistics.
We follow previous analyses by segmenting a bivariate linear regression model for
the house values.

The data set is available in package \pkg{mlbench} and can be obtained and transformed
via:
%
<<BostonHousing-data>>=
data("BostonHousing", package = "mlbench")
BostonHousing <- transform(BostonHousing,
  chas = factor(chas, levels = 0:1, labels = c("no", "yes")),
  rad = factor(rad, ordered = TRUE))
@
%
It provides $n = \Sexpr{NROW(BostonHousing)}$ observations of the median value of owner-occupied
homes in Boston (in USD~1000) along with $\Sexpr{NCOL(BostonHousing)}$ covariates including in particular
the number of rooms per dwelling (\code{rm}) and the percentage
of lower status of the population (\code{lstat}). A segment-wise linear relationship between
the value and these two variables is very intuitive, whereas the shape of the influence
of the remaining covariates is rather unclear and hence should be learned from the data.
Therefore, a linear regression model for median value explained by \verb:rm^2:
and \verb:log(lstat): is employed and partitioned with respect to all remaining variables.
Choosing appropriate
transformations of the dependent variable and the regressors that enter the linear
regression model is important to obtain a well-fitting model in each segment and
we follow in our choice the recommendations of \cite{Breiman+Friedman:1985}. Monotonic
transformations of the partitioning variables do not affect the recursive partitioning
algorithm and hence do not have to be performed. However, it is important to distinguish
between numerical and categorical variables for choosing an appropriate parameter 
stability test. The variable \code{chas} is a dummy indicator variable (for tract bounds
with Charles river) and thus needs to be turned into a factor. Furthermore, the variable
\code{rad} is an index of accessibility to radial highways and takes only 9 distinct
values. Hence, it is most appropriately treated as an ordered factor.
Note that both transformations only affect the parameter stability test chosen (step~2), not the splitting
procedure (step~3).
% Note that with splittry = 0 (according to old version of mob) there is no
% split in dis
<<BostonHousing-tree>>=
bh_tree <- lmtree(medv ~ log(lstat) + I(rm^2) | zn + indus + chas + nox +
  age + dis + rad + tax + crim + b + ptratio, data = BostonHousing)
bh_tree
@
%
The corresponding visualization is shown in Figure~\ref{fig:BostonHousing}.
It shows partial scatter plots of the dependent variable against each of the regressors
in the terminal nodes. Each scatter plot also shows the fitted values, i.e., a projection
of the fitted hyperplane.

\setkeys{Gin}{width=\textwidth}
\begin{figure}[p!]
\centering
<<BostonHousing-plot,echo=FALSE,fig.height=8,fig.width=12,out.extra='width=18cm,keepaspectratio,angle=90'>>=
plot(bh_tree)
@
\caption{\label{fig:BostonHousing} Linear-regression-based tree for the Boston housing data.
The plots in the leaves give partial scatter plots for \code{rm} (upper panel) and 
\code{lstat} (lower panel).}
\end{figure}

From this visualization, it can be seen that in the nodes~4, 6, 7 and 8 the increase of
value with the number of rooms dominates the picture (upper panel) whereas in node~9 the
decrease with the lower status population percentage (lower panel) is more pronounced. 
Splits are performed in the variables \code{tax} (poperty-tax rate) and
\code{ptratio} (pupil-teacher ratio).

For summarizing the quality of the fit, we could compute the mean squared error, log-likelihood
or AIC:
%
<<BostonHousing-AIC>>=
mean(residuals(bh_tree)^2)
logLik(bh_tree)
AIC(bh_tree)
@

\subsection{Teaching ratings data}

\cite{Hamermesh+Parker:2005} investigate the
correlation of beauty and teaching evaluations for professors.
They provide data
on course evaluations, course characteristics, and professor
characteristics for 463 courses for the academic years 2000--2002 at the
University of Texas at Austin. It is of interest how the average teaching
evaluation per course (on a scale 1--5) depends on a standardized measure
of beauty (as assessed by a committee of six persons based on photos).
\cite{Hamermesh+Parker:2005} employ a linear regression, weighted
by the number of students per course and adjusting for several further
main effects: gender, whether or not the teacher is from a minority,
a native speaker, or has tenure, respectively, and whether the course
is taught in the upper or lower division. Additionally, the age of the
professors is available as a regressor but not considered by
\cite{Hamermesh+Parker:2005} because the corresponding main effect is
not found to be significant in either linear or quadratic form.

Here, we employ a similar model but use the available regressors differently.
The basic model is again a linear regression for teaching rating by
beauty, estimated by weighted least squares (WLS). All remaining explanatory
variables are \emph{not} used as regressors but as partitioning variables
because we argue that it is unclear how they influence the correlation
between teaching rating and beauty. Hence, MOB is used to adaptively
incorporate these further variables and determine potential interactions.

First, the data are loaded from the \pkg{AER} package \citep{AER} and
only the subset of single-credit courses is excluded. 
%
<<TeachingRatings-data>>=
data("TeachingRatings", package = "AER")
tr <- subset(TeachingRatings, credits == "more")
@
%
The single-credit courses include elective modules that are quite different
from the remaining courses (e.g., yoga, aerobics, or dance) and are hence
omitted from the main analysis.

WLS estimation of the null model (with only an intercept) and the main
effects model is carried out in a first step:
%
<<TeachingRatings-lm>>=
tr_null <- lm(eval ~ 1, data = tr, weights = students)
tr_lm <- lm(eval ~ beauty + gender + minority + native + tenure + division,
  data = tr, weights = students)
@
%
Then, the model-based tree can be estimated with \code{lmtree()} using only
\code{beauty} as a regressor and all remaining variables for partitioning.
For WLS estimation, we set \code{weights = students} and \code{caseweights = FALSE}
because the weights are only proportionality factors and do not signal exactly
replicated observations \citep[see][for a discussion of the different types of weights]{Lumley:2020}.
%
<<TeachingRatings-tree>>=
(tr_tree <- lmtree(eval ~ beauty | minority + age + gender + division +
  native + tenure, data = tr, weights = students, caseweights = FALSE))
@
%
\begin{figure}[t!]
\setkeys{Gin}{width=\textwidth}
<<TeachingRatings-plot,echo=FALSE,results='hide',fig.height=8,fig.width=12>>=
plot(tr_tree)
@
\caption{\label{fig:tr_tree} WLS-based tree for the teaching ratings data.
The plots in the leaves show scatterplots for teaching rating by beauty score.}
\end{figure}
%
The resulting tree can be visualized by \code{plot(tr_tree)} and is shown in
Figure~\ref{fig:tr_tree}. This shows that despite age not having a significant
main effect \citep[as reported by][]{Hamermesh+Parker:2005}, it clearly plays
an important role: While the correlation of teaching rating and beauty score
is rather moderate for younger professors, there is a clear correlation for
older professors (with the cutoff age somewhat lower for female professors).
%
<<TeachingRatings-coef>>=
coef(tr_lm)[2]
coef(tr_tree)[, 2]
@
%
Th $R^2$ of the tree is also clearly improved over the main effects model:
%
<<TeachingRatings-rsquared>>=
1 - c(deviance(tr_lm), deviance(tr_tree))/deviance(tr_null)
@


\subsection{Titanic survival data}

To illustrate how differences in treatment effects can be captured by MOB,
the Titanic survival data is considered: The question is whether ``women
and children first'' is applied in the same way for all subgroups of the
passengers of the Titanic. Or, in other words, whether the effectiveness of
preferential treatment for women/children differed across subgroups.

The \code{Titanic} data is provided in base \proglang{R} as a contingency
table and transformed here to a `\code{data.frame}' for use with \code{glmtree()}:
%
<<Titanic-data>>=
data("Titanic", package = "datasets")
ttnc <- as.data.frame(Titanic)
ttnc <- ttnc[rep(1:nrow(ttnc), ttnc$Freq), 1:4]
names(ttnc)[2] <- "Gender"
ttnc <- transform(ttnc, Treatment = factor(
  Gender == "Female" | Age == "Child", levels = c(FALSE, TRUE),
  labels = c("Male&Adult", "Female|Child")))
@
%
The data provides factors \code{Survived} (yes/no), \code{Class} (1st,
2nd, 3rd, crew), \code{Gender} (male, female), and \code{Age} (child, adult).
Additionally, a factor \code{Treatment} is added that distinguishes women/children
from male adults.

To investigate how the preferential treatment effect (\code{Survived ~ Treatment})
differs across the remaining explanatory variables, the following
logistic-regression-based tree is considered. The significance
level of \code{alpha = 0.01} is employed here to avoid overfitting and separation
problems in the logistic regression.
%
<<Titanic-tree>>=
ttnc_tree <- glmtree(Survived ~ Treatment | Class + Gender + Age,
  data = ttnc, family = binomial, alpha = 0.01)
ttnc_tree
@
%
\begin{figure}[t!]
\setkeys{Gin}{width=\textwidth}
<<Titanic-plot,echo=FALSE,results='hide',fig.height=6,fig.width=10>>=
plot(ttnc_tree, tp_args = list(ylines = 1, margins = c(1.5, 1.5, 1.5, 2.5)))
@
\caption{\label{fig:ttnc_tree} Logistic-regression-based tree for the 
Titanic survival data. The plots in the leaves give spinograms for survival
status versus preferential treatment (women or children).}
\end{figure}
%
This shows that the treatment differs strongly across passengers classes, see also
the result of \code{plot(ttnc_tree)} in Figure~\ref{fig:ttnc_tree}. 
The treatment effect is much lower in the 3rd class where women/children still have
higher survival rates than adult men but the odds ratio is much lower compared to
all remaining classes. The split between the 2nd and the remaining two classes (1st, crew)
is due to a lower overall survival rate (intercepts of \Sexpr{round(coef(ttnc_tree)[2, 1], digits = 2)} and 
\Sexpr{round(coef(ttnc_tree)[3, 1], digits = 2)}, respectively) while the odds ratios
associated with the preferential treatment are rather similar
(\Sexpr{round(coef(ttnc_tree)[2, 2], digits = 2)} and \Sexpr{round(coef(ttnc_tree)[3, 2], digits = 2)},
respectively).

Another option for assessing the class effect would be to immediately split into
all four classes rather than using recursive binary splits. This can be obtained
by setting \code{catsplit = "multiway"} in the \code{glmtree()} call above. This
yields a tree with just a single split but four kid nodes.


\subsection{German breast cancer data}

To illustrate that the MOB approach can also be used beyond (generalized) linear
regression models, it is employed in the following to analyze censored survival
times among German women with positive node breast cancer. The data is available
in the \pkg{TH.data} package and the survival time is transformed from days to years:
%
<<GBSG2>>=
data("GBSG2", package = "TH.data")
GBSG2$time <- GBSG2$time/365
@
%
For regression a parametric Weibull regression based on the \code{survreg()} function
from the \pkg{survival} package \citep{pkg:survival} is used. A fitting function for
\code{mob()} can be easily set up:
%
<<wbreg, results='hide'>>=
library("survival")
wbreg <- function(y, x, start = NULL, weights = NULL, offset = NULL, ...) {
  survreg(y ~ 0 + x, weights = weights, dist = "weibull", ...)
}
@
%
As the \pkg{survreg} package currently does not provide a \code{logLik()}
method for `\code{survreg}' objects, this needs to be added here:
%
<<logLik.survreg>>=
logLik.survreg <- function(object, ...)
  structure(object$loglik[2], df = sum(object$df), class = "logLik")
@
%
Without the \code{logLik()} method, \code{mob()} would not know how to extract
the optimized objective function from the fitted model.

With the functions above available, a censored Weibull-regression-tree can
be fitted: The dependent variable is the censored survival time and the
two regressor variables are the main risk factor (number of positive lymph nodes)
and the treatment variable (hormonal therapy). All remaining variables are
used for partitioning: age, tumor size and grade, progesterone and estrogen receptor,
and menopausal status. The minimal segment size is set to 80.
%
<<gbsg2_tree>>=
gbsg2_tree <- mob(Surv(time, cens) ~ horTh + pnodes | age + tsize +
  tgrade + progrec + estrec + menostat, data = GBSG2,
  fit = wbreg, control = mob_control(minsize = 80))
@
%
\begin{figure}[p!]
\centering
\setkeys{Gin}{width=0.6\textwidth}
<<GBSG2-plot, echo=FALSE, results='hide', fig.height=4.5, fig.width=5>>=
plot(gbsg2_tree)
@
\caption{\label{fig:GBSG2} Censored Weibull-regression-based tree for the German
breast cancer data. The plots in the leaves report the estimated regression coefficients.}

\setkeys{Gin}{width=\textwidth}
<<GBSG2-scatter, echo=FALSE, results='hide', fig.height=6, fig.width=9>>=
gbsg2node <- function(mobobj, 
  col = "black", linecol = "red", cex = 0.5, pch = NULL,
  jitter = FALSE, xscale = NULL, yscale = NULL, ylines = 1.5,
  id = TRUE, xlab = FALSE, ylab = FALSE)
{
  ## obtain dependent variable
  mf <- model.frame(mobobj)
  y <- Formula::model.part(mobobj$info$Formula, mf, lhs = 1L, rhs = 0L)
  if(isTRUE(ylab)) ylab <- names(y)[1L]
  if(identical(ylab, FALSE)) ylab <- ""
  if(is.null(ylines)) ylines <- ifelse(identical(ylab, ""), 0, 2)
  y <- y[[1L]]

  ## plotting character and response
  if(is.null(pch)) pch <- y[,2] * 18 + 1
  y <- y[,1]
  y <- as.numeric(y)
  pch <- rep(pch, length.out = length(y))
  if(jitter) y <- jitter(y)

  ## obtain explanatory variables
  x <- Formula::model.part(mobobj$info$Formula, mf, lhs = 0L, rhs = 1L)
  xnam <- colnames(x)
  z <- seq(from = min(x[,2]), to = max(x[,2]), length = 51)
  z <- data.frame(a = rep(sort(x[,1])[c(1, NROW(x))], c(51, 51)), b = z)
  names(z) <- names(x)
  z$x <- model.matrix(~ ., data = z)
  
  ## fitted node ids
  fitted <- mobobj$fitted[["(fitted)"]]
      
  if(is.null(xscale)) xscale <- range(x[,2]) + c(-0.1, 0.1) * diff(range(x[,2]))
  if(is.null(yscale)) yscale <- range(y) + c(-0.1, 0.1) * diff(range(y))
       
  ## panel function for scatter plots in nodes
  rval <- function(node) {
  
    ## node index
    nid <- id_node(node)
    ix <- fitted %in% nodeids(mobobj, from = nid, terminal = TRUE)

    ## dependent variable
    y <- y[ix]

    ## predictions
    yhat <- if(is.null(node$info$object)) {
      refit.modelparty(mobobj, node = nid)
    } else {
      node$info$object
    }
    yhat <- predict(yhat, newdata = z, type = "quantile", p = 0.5)
    pch <- pch[ix]

    ## viewport setup
    top_vp <- viewport(layout = grid.layout(nrow = 2, ncol = 3,
        	       widths = unit(c(ylines, 1, 1), c("lines", "null", "lines")),  
  		       heights = unit(c(1, 1), c("lines", "null"))),
        	       width = unit(1, "npc"), 
        	       height = unit(1, "npc") - unit(2, "lines"),
  		       name = paste("node_scatterplot", nid, sep = ""))
    pushViewport(top_vp)
    grid.rect(gp = gpar(fill = "white", col = 0))

    ## main title
    top <- viewport(layout.pos.col = 2, layout.pos.row = 1)
    pushViewport(top)
    mainlab <- paste(ifelse(id, paste("Node", nid, "(n = "), ""),
  		     info_node(node)$nobs, ifelse(id, ")", ""), sep = "")
    grid.text(mainlab)
    popViewport()

    plot_vp <- viewport(layout.pos.col = 2, layout.pos.row = 2, xscale = xscale,
  	yscale = yscale, name = paste("node_scatterplot", nid, "plot", sep = ""))
    pushViewport(plot_vp)

    ## scatterplot
    grid.points(x[ix,2], y, gp = gpar(col = col, cex = cex), pch = pch)
    grid.lines(z[1:51,2], yhat[1:51], default.units = "native", gp = gpar(col = linecol))
    grid.lines(z[52:102,2], yhat[52:102], default.units = "native", gp = gpar(col = linecol, lty = 2))

    grid.xaxis(at = c(ceiling(xscale[1]*10), floor(xscale[2]*10))/10)
    grid.yaxis(at = c(ceiling(yscale[1]), floor(yscale[2])))

    if(isTRUE(xlab)) xlab <- xnam[2]
    if(!identical(xlab, FALSE)) grid.text(xlab, x = unit(0.5, "npc"), y = unit(-2, "lines"))
    if(!identical(ylab, FALSE)) grid.text(ylab, y = unit(0.5, "npc"), x = unit(-2, "lines"), rot = 90)

    grid.rect(gp = gpar(fill = "transparent"))
    upViewport()

    upViewport()
  }
          
  return(rval)
}
class(gbsg2node) <- "grapcon_generator"

plot(gbsg2_tree, terminal_panel = gbsg2node, tnex = 2, 
  tp_args = list(xscale = c(0, 52), yscale = c(-0.5, 8.7)))
@
\caption{\label{fig:GBSG2-scatter} Censored Weibull-regression-based tree for the German
breast cancer data. The plots in the leaves depict censored (hollow) and uncensored
(solid) survival time by number of positive lymph nodes along with fitted median
survival for patients with (dashed line) and without (solid line) hormonal therapy.}
\end{figure}
%
Based on progesterone receptor, a tree with two leaves is found:
%
<<gbsg2_tree-methods>>=
gbsg2_tree
coef(gbsg2_tree)
logLik(gbsg2_tree)
@
%
The visualization produced by \code{plot(gbsg2_tree)} is shown in Figure~\ref{fig:GBSG2}.
A nicer graphical display using a scatter plot (with indication of censoring) and
fitted regression curves is shown in Figure~\ref{fig:GBSG2-scatter}. This uses a
custom panel function whose code is too long and elaborate to be shown here.
Interested readers are referred to the \proglang{R} code underlying the vignette.

The visualization shows that for higher progesterone receptor levels: (1)~survival times are
higher overall, (2)~the treatment effect of hormonal therapy is higher, and
(3)~the negative effect of the main risk factor (number of positive lymph nodes) is less severe.



\section{Setting up a new mobster} \label{sec:mobster}

To conclude this vignette, we present another illustration that shows how
to set up new mobster functionality from scratch. To do so, we implement the
Bradley-Terry tree suggested by \cite{Strobl+Wickelmaier+Zeileis:2011} ``by hand''.
The \pkg{psychotree} package already provides an easy-to-use mobster called
\code{bttree()} but as an implementation exercise we recreate its functionality
here.

The only inputs required are a suitable data set with paired comparisons
(\code{Topmodel2007} from \pkg{psychotree}) and a parametric model for paired
comparison data (\code{btmodel()} from \pkg{psychotools}, implementing the
Bradley-Terry model). The latter optionally computes the empirical estimating functions
and already comes with a suitable extractor method.
<<bt-packages>>=
data("Topmodel2007", package = "psychotree")
library("psychotools")
@
%
The Bradley-Terry (or Bradley-Terry-Luce) model is a standard model for paired
comparisons in social sciences. It parametrizes the probability $\pi_{ij}$
for preferring some object $i$ over another object $j$ in terms of corresponding
``ability'' or ``worth'' parameters $\theta_i$:
\begin{eqnarray*}
      \pi_{ij}\phantom{)}  & = & \frac{\theta_i}{\theta_i + \theta_j} \\
  \mathsf{logit}(\pi_{ij}) & = & \log(\theta_i) - \log(\theta_j)
\end{eqnarray*}
This model can be easily estimated by maximum likelihood as a logistic or
log-linear GLM. This is the approach used internally by \code{btmodel()}.

The \code{Topmodel2007} data provide paired comparisons of attractiveness among
the six finalists of the TV show \emph{Germany's Next Topmodel~2007}:
Barbara, Anni, Hana, Fiona, Mandy, Anja. The data were collected in a
survey with 192~respondents at Universit{\"a}t T{\"u}bingen and the available
covariates comprise gender, age, and familiarty with the TV~show. The latter
is assess by three by yes/no questions:
(1)~Do you recognize the women?/Do you know the show?
(2)~Did you watch it regularly?
(3)~Did you watch the final show?/Do you know who won?

To fit the Bradley-Terry tree to the data, the available building blocks
just have to be tied together. First, we set up the basic/simple model fitting
interface described in Section~\ref{sec:fit}:
%
<<btfit1>>=
btfit1 <- function(y, x = NULL, start = NULL, weights = NULL,
  offset = NULL, ...) btmodel(y, ...)
@
%
The function \code{btfit1()} simply calls \code{btmodel()} ignoring all arguments
except \code{y} as the others are not needed here. No more processing is required
because \class{btmodel} objects come with a \code{coef()}, \code{logLik()}, and
\code{estfun()} method. Hence, we can call \code{mob()}
now specifying the response and the partitioning variable (and no regressors because
there are no regressors in this model).
%
<<bt1>>=
bt1 <- mob(preference ~ 1 | gender + age + q1 + q2 + q3,
  data = Topmodel2007, fit = btfit1)
@
%
An alternative way to fit the exact same tree somewhat more quickly would be to
employ the extended interface described in Section~\ref{sec:fit}:
%
<<btfit2>>=
btfit2 <- function(y, x = NULL, start = NULL, weights = NULL,
  offset = NULL, ..., estfun = FALSE, object = FALSE) {
  rval <- btmodel(y, ..., estfun = estfun, vcov = object)
  list(
    coefficients = rval$coefficients,
    objfun = -rval$loglik,
    estfun = if(estfun) rval$estfun else NULL,
    object = if(object) rval else NULL
  )
}
@
%
Still \code{btmodel()} is employed for fitting the model but the quantities
\code{estfun} and \code{vcov} are only computed if they are really required. This
may save some computation time -- about 20\% on the authors' machine at the time of
writing -- when computing the tree:
%
<<bt2>>=
bt2 <- mob(preference ~ 1 | gender + age + q1 + q2 + q3,
  data = Topmodel2007, fit = btfit2)
@
%
The speed-up is not huge but comes almost for free: just a few additional lines
of code in \code{btfit2()} are required. For other models where it is more costly
to set up a full model (with variance-covariance matrix, predictions, residuals, etc.)
larger speed-ups are also possible.

Both trees, \code{bt1} and \code{bt2}, are equivalent (except for the details of the
fitting function). Hence, in the following we only explore \code{bt2}. However, the
same code can be applied to \code{bt1} as well.

Many tools come completely for free and are inherited from the general \class{modelparty}/\class{party}.
For example, both printing (\code{print(bt2)}) and plotting (\code{plot(bt2)})
shows the estimated parameters in the terminal nodes which can also be extracted by
the \code{coef()} method:
<<bt2-print>>=
bt2
coef(bt2)
@
The corresponding visualization is shown in the upper panel of Figure~\ref{fig:topmodel-plot1}.
In all cases, the estimated coefficients on the logit scale omitting the
fixed zero reference level (Anja) are reported. To show the corresponding worth
parameters $\theta_i$ including the reference level, one can simply provide
a small panel function \code{worthf()}. This applies the \code{worth()} function
from \pkg{psychotools} to the fitted-model object stored in the \code{info} slot
of each node, yielding the lower panel in Figure~\ref{fig:topmodel-plot1}.
%
<<bt2-worthf, eval=FALSE>>=
worthf <- function(info) paste(info$object$labels,
  format(round(worth(info$object), digits = 3)), sep = ": ")
plot(bt2, FUN = worthf)
@
%
\begin{figure}[p!]
\centering
<<bt2-plot, fig.height=7.4, fig.width=12, echo=FALSE>>=
plot(bt2)
@

\vspace*{0.5cm}

<<bt2-plot2, fig.height=7.4, fig.width=12, echo=FALSE>>=
<<bt2-worthf>>
@
\caption{\label{fig:topmodel-plot1} Bradley-Terry-based tree for the topmodel attractiveness
data. The default plot (upper panel) reports the estimated coefficients on the log scale
while the adapted plot (lower panel) shows the corresponding worth parameters.}
\end{figure}
%
To show a graphical display of these worth parameters rather than
printing their numerical values, one can use a simply glyph-style plot.
A simply way to generate these is to use the \code{plot()} method for
\class{btmodel} objects from \pkg{partykit} and \code{nodeapply} this to
all terminal nodes (see Figure~\ref{fig:topmodel-plot2}):
%
<<bt2-nodeapply, eval=FALSE>>=
par(mfrow = c(2, 2))
nodeapply(bt2, ids = c(3, 5, 6, 7), FUN = function(n)
  plot(n$info$object, main = n$id, ylim = c(0, 0.4)))
@
%
\begin{figure}[t!]
\centering
<<bt2-nodeapply-plot, fig.height=7, fig.width=9, echo=FALSE, results='hide'>>=
<<bt2-nodeapply>>
@
\caption{\label{fig:topmodel-plot2} Worth parameters in the terminal nodes
of the Bradley-Terry-based tree for the topmodel attractiveness data.}
\end{figure}
%
Alternatively, one could set up a proper panel-generating function in \pkg{grid}
that allows to create the glyphs within the terminal nodes of the tree (see
Figure~\ref{fig:topmodel-plot3}). As the code for this panel-generating function
\code{node_btplot()} is too complicated to display within the vignette, we
refer interested readers to the underlying code. Given this panel-generating
function Figure~\ref{fig:topmodel-plot3} can be generated with
<<bt2-plot3, eval=FALSE>>=
plot(bt2, drop = TRUE, tnex = 2,
  terminal_panel = node_btplot(bt2, abbreviate = 1, yscale = c(0, 0.5)))
@

\begin{figure}[t!]
\centering
<<node_btplot, echo=FALSE, results='hide', fig.height=6.2, fig.width=10>>=
## visualization function
node_btplot <- function(mobobj, id = TRUE,
  worth = TRUE, names = TRUE, abbreviate = TRUE, index = TRUE, ref = TRUE,
  col = "black", linecol = "lightgray", cex = 0.5, pch = 19, xscale = NULL, yscale = NULL, ylines = 1.5)
{
    ## node ids
    node <- nodeids(mobobj, terminal = FALSE)
    
    ## get all coefficients 
    cf <- partykit:::apply_to_models(mobobj, node, FUN = function(z)        
      if(worth) worth(z) else coef(z, all = FALSE, ref = TRUE))
    cf <- do.call("rbind", cf)
    rownames(cf) <- node

    ## get one full model
    mod <- partykit:::apply_to_models(mobobj, node = 1L, FUN = NULL)

    if(!worth) {
      if(is.character(ref) | is.numeric(ref)) {
        reflab <- ref
        ref <- TRUE
      } else {
        reflab <- mod$ref
      }
      if(is.character(reflab)) reflab <- match(reflab, mod$labels)
      cf <- cf - cf[,reflab]
    }

    ## reference
    if(worth) {
      cf_ref <- 1/ncol(cf)
    } else {
      cf_ref <- 0
    }

    ## labeling
    if(is.character(names)) {
      colnames(cf) <- names
      names <- TRUE
    }

    ## abbreviation
    if(is.logical(abbreviate)) {
      nlab <- max(nchar(colnames(cf)))
      abbreviate <- if(abbreviate) as.numeric(cut(nlab, c(-Inf, 1.5, 4.5, 7.5, Inf))) else nlab
    }
    colnames(cf) <- abbreviate(colnames(cf), abbreviate)
    
    if(index) {
      x <- 1:NCOL(cf)
      if(is.null(xscale)) xscale <- range(x) + c(-0.1, 0.1) * diff(range(x))
    } else {
      x <- rep(0, length(cf))
      if(is.null(xscale)) xscale <- c(-1, 1)      
    }
    if(is.null(yscale)) yscale <- range(cf) + c(-0.1, 0.1) * diff(range(cf))
         
    ## panel function for bt plots in nodes
    rval <- function(node) {

      ## node index
      id <- id_node(node)
    
      ## dependent variable setup
      cfi <- cf[id,]

      ## viewport setup
      top_vp <- viewport(layout = grid.layout(nrow = 2, ncol = 3,
    			 widths = unit(c(ylines, 1, 1), c("lines", "null", "lines")),  
        		 heights = unit(c(1, 1), c("lines", "null"))),
    			 width = unit(1, "npc"), 
    			 height = unit(1, "npc") - unit(2, "lines"),
        		 name = paste("node_btplot", id, sep = ""))
      pushViewport(top_vp)
      grid.rect(gp = gpar(fill = "white", col = 0))

      ## main title
      top <- viewport(layout.pos.col = 2, layout.pos.row = 1)
      pushViewport(top)
      mainlab <- paste(ifelse(id, paste("Node", id, "(n = "), ""),
        	       info_node(node)$nobs, ifelse(id, ")", ""), sep = "")
      grid.text(mainlab)
      popViewport()

      ## actual plot  
      plot_vpi <- viewport(layout.pos.col = 2, layout.pos.row = 2,
        xscale = xscale, yscale = yscale, 
        name = paste("node_btplot", id, "plot", sep = ""))
      pushViewport(plot_vpi)

      grid.lines(xscale, c(cf_ref, cf_ref), gp = gpar(col = linecol), default.units = "native")
      if(index) {
        grid.lines(x, cfi, gp = gpar(col = col, lty = 2), default.units = "native")
        grid.points(x, cfi, gp = gpar(col = col, cex = cex), pch = pch, default.units = "native")
        grid.xaxis(at = x, label = if(names) names(cfi) else x)
      } else {  	
        if(names) grid.text(names(cfi), x = x, y = cfi, default.units = "native")
          else grid.points(x, cfi, gp = gpar(col = col, cex = cex), pch = pch, default.units = "native")
      }
      grid.yaxis(at = c(ceiling(yscale[1] * 100)/100, floor(yscale[2] * 100)/100))
      grid.rect(gp = gpar(fill = "transparent"))

      upViewport(2)
    }
	    
    return(rval)
}
class(node_btplot) <- "grapcon_generator"

plot(bt2, drop = TRUE, tnex = 2,
  terminal_panel = node_btplot(bt2, abbreviate = 1, yscale = c(0, 0.5)))
@
\caption{\label{fig:topmodel-plot3} Bradley-Terry-based tree for the topmodel attractiveness
data with visualizations of the worth parameters in the terminal nodes.}
\end{figure}

Finally, to illustrate how different predictions can be easily computed,
we set up a small data set with three new individuals:
%
<<tm, echo=FALSE>>=
tm <- data.frame(age = c(60, 25, 35), gender = c("male", "female", "female"),
  q1 = "no", q2 = c("no", "no", "yes"), q3 = "no")
tm
@
%
For these we can easily compute (1)~the predicted node ID, (2)~the corresponding
worth parameters, (3)~the associated ranks.
<<tm-predict>>=
tm
predict(bt2, tm, type = "node")
predict(bt2, tm, type = function(object) t(worth(object)))
predict(bt2, tm, type = function(object) t(rank(-worth(object))))
@

This completes the tour of fitting, printing, plotting, and predicting the
Bradley-Terry tree model. Convenience interfaces that employ code like shown
above can be easily defined and collected in new packages such as \pkg{psychotree}.


\section{Conclusion}
\label{sec:conclusion}

The function \code{mob()} in the \pkg{partykit} package provides a new flexible and object-oriented
implementation of the general algorithm for model-based recursive partitioning using
\pkg{partykit}'s recursive partytioning infrastructure. New model
fitting functions can be easily provided, especially if standard extractor functions
(such as \code{coef()}, \code{estfun()}, \code{logLik()}, etc.) are available. The resulting
model trees can then learned, visualized, investigated, and employed for predictions.
To gain some efficiency in the computations and to allow for further customization
(in particular specialized visualization and prediction methods), convenience interfaces
\code{lmtree()} and \code{glmtree()} are provided for recursive partitioning based on
(generalized) linear models.


\bibliography{\Sexpr{gsub("\\.bib", "", system.file("REFERENCES.bib", package = "partykit"))},packages}

\end{document}
