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\def\tree{\texttt{tree}}
\def\rpart{\texttt{rpart}}
\def\splus{S-Plus}
\newcommand{\Co}[1]{\texttt{#1}}
\title {An Introduction to Recursive Partitioning\\
Using the RPART Routines}
\author{Terry M. Therneau \\
Elizabeth J. Atkinson\\
Mayo Foundation }
\date{\today}
\begin{document}
\maketitle
\tableofcontents
<>=
options(continue = " ", width = 60)
options(SweaveHooks=list(fig=function() par(mar = c(4.1, 4.1, 0.1, 1.1))))
pdf.options(pointsize = 10)
par(xpd = NA) #stop clipping
library(rpart)
@
\section{Introduction}
This document is a modification of a technical report from the
Mayo Clinic Division of Biostatistics \cite{Therneau97},
which was itself an expansion of an earlier Stanford report
\cite{Therneau83}.
It is intended to give a short overview of
the methods found in the {\rpart} routines, which implement many of the
ideas found in the CART (Classification and Regression Trees) book and
programs of Breiman, Friedman, Olshen and Stone \cite{Breiman83}.
Because CART is the trademarked name of a particular software
implementation of these ideas, and \emph{tree} has been used
for the {\splus} routines of Clark and Pregibon~\cite{Clark92}
a different acronym
--- Recursive PARTitioning or rpart --- was chosen.
It is somewhat humorous that this label ``rpart'' has now become more
common than the original and more descriptive ``cart'',
a testament to the
influence of freely available software.
The {\rpart} programs build classification or regression models of a very
general structure using a two stage procedure; the resulting models can
be represented as binary trees. An example is some preliminary data
gathered at Stanford on revival of cardiac arrest patients by
paramedics. The goal is to predict which patients can be successfully
revived in the
field on the basis of fourteen variables known at or near the time of
paramedic arrival, e.g., sex, age, time from attack to first care, etc.
Since some patients who are not revived on site are later successfully
resuscitated at the hospital, early identification of these ``recalcitrant''
cases is of considerable clinical interest.
\begin{figure}
\begin{picture}(350, 160)(-40,0)
\thicklines
\put(120,160){\phantom{2}24 revived}
\put(120,150){144 not revived}
\put(150,130){\line( 2,-1){89}}
\put(150,130){\line(-2,-1){89}}
\put(50, 70){$X_1=1$}
\put(50, 60){22 / 13}
\put(210, 70){$X_1=$2, 3 or 4}
\put(220, 60){2 / 131}
\put(60, 54) {\line(-2,-1){45}}
\put(60, 54) {\line( 2,-1){45}}
\put(240, 54){\line( 2,-1){45}}
\put(240, 54){\line(-2,-1){45}}
\put( 5,10){$X_2=1$}
\put( 5, 0){20 / 5}
\put( 90,10){$X_2=$2 or 3}
\put( 95, 0){2 / 8}
\put(185,10){$X_3=$1}
\put(185, 0){2 / 31}
\put(270,10){$X_3=$2 or 3}
\put(275, 0){0 / 100}
\end{picture}
\caption{Revival data}
\label{revival}
\end{figure}
The resultant model separated the patients into four groups as shown in
figure \ref{revival}, where
\begin{tabbing}
\qquad \qquad \= $X1 $ = \= initial heart rhythm \\
\> \> 1= VF/VT 2=EMD 3=Asystole 4=Other \\ \\
\> $X_2 $ = initial response to defibrillation \\
\> \>1=Improved 2=No change 3=Worse \\ \\
\> $X_3 $ = initial response to drugs \\
\> \> 1=Improved 2=No change 3=Worse
\end{tabbing}
The other 11 variables did not appear in the final model.
This procedure seems to work especially well for variables such as
$X_1 $ ,
where there is a definite ordering, but spacings are not necessarily equal.
The tree is built by the following process: first the single variable
is found which best splits the data into two groups (`best' will be defined
later). The data is separated, and then this process is applied
{\em separately}
to each sub-group, and so on recursively until the subgroups either reach
a minimum size (5 for this data) or until no improvement can be made.
The resultant model is, with a certainty, too complex, and the question
arises as it does with all stepwise procedures of when to stop. The
second stage of the procedure consists of using cross-validation to
trim back the full tree. In the medical example above the
full tree had ten terminal regions. A cross validated estimate of risk
was computed for a nested set of sub trees; this final model was that sub tree
with the lowest estimate of risk.
\section{Notation}
The partitioning method can be applied to many different kinds of
data. We will start by looking at the classification problem,
which is one of the more instructive cases (but also has the
most complex equations).
The sample population consists of $n$ observations from $C$ classes. A given
model will break these observations into $k$ terminal groups;
to each of these groups is assigned a predicted class.
In an actual application, most parameters will be estimated from
the data, such estimates are given by $\approx$ formulae.
\begin{quote}
\begin{tabbing}
$\pi_i $\qquad\qquad \=$i=1,2,...,C$ \qquad prior probabilities of each class \\
\\
$L(i,j)$ \> $i=1,2,...,C$ \quad Loss matrix for incorrectly classifying \\
\> an $i$ as a $j$. $L(i,i)\equiv 0 $ \\
\\
$A$ \>some node of the tree \\
\> Note that A represents both a set of individuals in \\
\> the sample data, and, via the tree that produced it, \\
\> a classification rule for future data. \\
\\
$\tau(x)$ \> true class of an observation $ x$, where $x$ is the \\
\> vector of predictor variables \\
\\
$\tau( A ) $ \>
the class assigned to A, if A were to be taken as a \\
\> final node \\
\\
$n_i, \,n_A$ \>
number of observations in the sample that are class $i$, \\
\> number of obs in node $A$ \\
\\
$P(A) $ \>probability of $A$ (for future observations) \\
\>$=\sum_{i=1}^C \pi_i \,P\{ x \in A \mid \tau( x) =i \}$ \\
\>$ \approx \sum_{i=1}^C \pi_i n_{iA} / n_i $ \\
\\
$p(i|A)$ \> $P\{ \tau( x) =i \mid x \in A \} $ (for future observations) \\
\>$= \pi_i P \{ x \in A\mid\tau( x)=i\} /P \{ x \in A\} $ \\
\>$ \approx \pi_i (n_{iA} / n_i) /\sum \pi_i (n_{iA} / n_i$ )\\
\\
$R(A) $ \>risk of $A$ \\
\> $= \sum_{i=1}^C p(i|A) L(i, \tau(A)) $ \\
\> where $\tau(A) $ is chosen to minimize this risk \\
\\
$R(T)$ \>risk of a model (or tree) T \\
\>$ =\sum_{j=1}^k P(A_j) R(A_j) $ \\
\> where $A_j $ are the terminal nodes of the tree \\
\end{tabbing}
\end{quote}
If $L(i,j)=1$ for all $i \ne j$, and we set the prior probabilities
$\pi$ equal to the observed class frequencies in the sample
then $p(i|A)= n_{iA}/n_A$ and
$R(T)$ is the proportion misclassified.
\section{Building the tree}
\subsection{Splitting criteria}
If we split a node $A$ into two sons $A_L $ and $A_R $
(left and right sons), we will have
$$
P(A_L ) r(A_L )+ P(A_R ) r(A_R ) \le P(A) r(A)
$$
(this is proven in \cite{Breiman83}).
Using this, one obvious way to build a tree is
to choose that split which maximizes $\Delta r$, the decrease in risk.
There are defects with this, however, as the following example shows:
\begin{quote}
Suppose losses are equal and that the data is 80\% class 1's, and that some
trial split results in $A_L $ being 54\% class 1's and
$A_R $ being 100\% class 1's.
Since the minimum risk prediction for both the left and right son is
$\tau (A_L ) = \tau (A_R ) =1$, this split will
have $ \Delta r=0$, yet
scientifically this is a very informative division of the sample.
In real data with such a majority, the first few splits very often can
do no better than this.
\end{quote}
A more serious defect is that the risk reduction is essentially linear.
If there were two competing splits, one separating the data into groups
of 85\% and 50\% purity respectively, and the other into 70\%-70\%, we would
usually prefer the former, if for no other reason than because it better
sets things up for the next splits.
One way around both of these problems is to use look-ahead rules; but
these are computationally very expensive. Instead {\rpart} uses one of
several measures of impurity, or diversity, of a node. Let $f$ be some
impurity function and define the impurity of a node A as
$$
I(A)= \sum_{i=1}^C f(p_{iA} )
$$
where $p_{iA} $ is the proportion of those in $A$ that belong to class
$i$ for future samples.
Since we would like $I(A)$ =0 when $A$ is pure, $f$ must be concave with
$f(0)=f(1)=0$.
Two candidates for $f$ are the information index $f(p)=-p\log(p)$ and
the Gini
index $f(p)=p(1-p)$. We then use that split with maximal impurity
reduction
$$
\Delta I =p(A)I(A)-p(A_L )I(A_L )-p(A_R )I(A_R )
$$
The two impurity functions are plotted in figure (\ref{ginip}),
along with a rescaled version of the Gini measure.
For the two class problem the measures differ only slightly, and
will nearly always choose the same split point.
\begin{figure}
\myfig{longintro-impurity}
\caption{Comparison of Gini and Information impurity for two groups.}
\label{ginip}
\end{figure}
<>=
ptemp <- seq(0, 1, length = 101)[2:100]
gini <- 2* ptemp *(1-ptemp)
inform <- -(ptemp*log(ptemp) + (1-ptemp)*log(1-ptemp))
sgini <- gini *max(inform)/max(gini)
matplot(ptemp, cbind(gini, inform, sgini), type = 'l', lty = 1:3,
xlab = "P", ylab = "Impurity", col = 1)
legend(.3, .2, c("Gini", "Information", "rescaled Gini"),
lty = 1:3, col = 1, bty = 'n')
@
Another convex criteria not quite of the above class is twoing for which
$$
I(A)=\min_{C_1C_2} [f(p_{C_1} )+f(p_{C_2} )]
$$
where $C_1 $,$C_2 $ is some partition of the $C$ classes into
two disjoint sets. If $C=2$ twoing is equivalent to the usual impurity index
for $f$.
Surprisingly, twoing can be calculated almost as efficiently as the
usual impurity index.
One potential advantage of twoing is that the output may give the user
additional insight concerning the structure of the data. It can be viewed
as the partition of $C$ into two superclasses which are in some sense the
most dissimilar for those observations in $A$.
For certain problems there may be a natural ordering of the response
categories (e.g. level of education), in which case ordered twoing can be
naturally defined, by restricting $C_1$ to be an interval $[1,2,\ldots,k]$
of classes.
Twoing is not part of {\rpart}.
\subsection{Incorporating losses}
One salutatory aspect of the risk reduction criteria not found
in the impurity measures is inclusion of the loss function. Two different
ways of extending the impurity criteria
to also include losses
are implemented in CART, the
generalized Gini index and altered priors.
The {\rpart} software implements only the altered priors method.
\subsubsection{Generalized Gini index}
The Gini index has the following interesting interpretation. Suppose
an object is selected at random from one of C classes according to the
probabilities $(p_1 ,p_2 ,...,p_C ) $
and is randomly assigned to a class using the same distribution. The
probability of misclassification is
$$
\sum_ i \sum_ {j \ne i} p_i p_j =
\sum_ i \sum_ j p_i p_j - \sum_ i p_i^2
= \sum_ i 1- p_i^2 = \hbox{Gini index for $p$}
$$
Let $L(i,j)$ be the loss of assigning class $j$ to an object which actually
belongs to class $i$. The expected cost of misclassification is
$\sum \sum L(i,j)p_i p_j $. This suggests defining a
{\em generalized Gini} index of impurity by
$$
G( p) = \sum_ i \sum_ j L(i,j)p_i p_j
$$
The corresponding splitting criterion appears to be promising for applications
involving variable misclassification costs. But there are several
reasonable objections to it. First, $G( p) $ is not necessarily a
concave function of $ p$, which was the motivating factor behind
impurity measures. More seriously, $G$ symmetrizes the loss
matrix before using it. To see this note that
$$
G( p) = (1/2) \sum \sum [L(i,j)+L(j,i)] \, p_i p_j
$$
In particular, for two-class problems, $G$ in effect ignores the
loss matrix.
\subsubsection{Altered priors}
Remember the definition of $R(A)$
\begin{eqnarray*}
R(A) &\equiv& \sum_{i=1}^C p_{iA}L(i, \tau (A)) \\
&=& \sum_{i=1}^C \pi_i L(i, \tau (A))
(n_{iA} / n_i )( n / n_A )
\end{eqnarray*}
Assume there exists $\tilde \pi $ and $\tilde L $ be such that
$$
\tilde \pi_i \tilde L (i,j) = \pi_i L(i,j)\qquad \forall i,j \in C
$$
Then $R(A)$ is unchanged under the new losses and priors. If $\tilde L$
is proportional to the zero-one loss matrix then the priors $\tilde \pi$
should be used in the splitting criteria. This is possible only if $L$ is
of the form
$$
L(i,j) = \left\{ \begin{array}{ll}
L_i & i \ne j \\
0 & i=j
\end{array} \right .
$$
in which case
$$
\tilde \pi_i = \frac{\pi_i L_i } { \sum_j \pi_j L_j }
$$
This is always possible when $C=2$,
and hence altered priors are exact for the two class problem.
For arbitrary loss matrix of dimension $C> 2$,
{\rpart} uses the above formula with
$L_i = \sum_ j L(i,j)$.
A second justification for altered priors is this. An impurity index
$I(A) = \sum f(p_i )$ has its maximum at $p_1= p_2 =\ldots =
p_C = 1/C$. If a problem had, for instance, a misclassification
loss for class 1 which was twice the loss for a class 2 or 3
observation, one would wish I(A) to have its maximum at $p_1 =$1/5,
$p_2 = p_3 =$2/5, since this is the worst possible set of proportions
on which to decide a node's class. The altered priors technique does
exactly this, by shifting the $p_i $.
Two final notes
\begin{itemize}
\item When altered priors are used, they affect only the choice of split.
The ordinary losses and priors are used to compute the risk of the node.
The altered priors simply help the impurity rule choose splits that
are likely to be ``good'' in terms of the risk.
\item The argument for altered priors is valid for both the Gini and
information splitting rules.
\end{itemize}
\subsection{Example: Stage C prostate cancer (\Co{class} method)}
\begin{figure}
\myfig{longintro-gini1}
\caption{Classification tree for the Stage C data}
\label{fgini1}
\end{figure}
This first example is based on a data set of 146 stage C prostate
cancer patients \cite{Nativ88}.
The main clinical endpoint of interest is whether the
disease recurs after initial surgical removal of the prostate, and
the time interval to that progression (if any).
The endpoint in this example is \Co{status}, which takes
on the value 1 if the disease has progressed and 0 if not.
Later we'll analyze
the data using the \Co{exp} method, which will take into account
time to progression. A short description of each of the variables is
listed below. The main predictor variable of interest in
this study was DNA ploidy, as determined by flow cytometry.
For diploid and tetraploid tumors, the flow cytometry method was
also able to estimate the percent of tumor cells in a $G_2$ (growth)
stage of their cell cycle; $G_2$\% is systematically missing
for most aneuploid tumors.
The variables in the data set are
\begin{center}
\begin{tabular}{ll}
pgtime & time to progression, or last follow-up free of progression \\
pgstat & status at last follow-up (1=progressed, 0=censored)\\
age & age at diagnosis \\
eet & early endocrine therapy (1=no, 0=yes) \\
ploidy & diploid/tetraploid/aneuploid DNA pattern \\
g2 & \% of cells in $G_2$ phase \\
grade & tumor grade (1-4) \\
gleason & Gleason grade (3-10) \\
\end{tabular}
\end{center}
The model is fit by using the {\rpart} function. The first
argument of the function is a model formula,
with the $\sim$ symbol standing for ``is modeled as''.
The \Co{print} function gives an abbreviated output, as for other R models.
The \Co{plot} and \Co{text} command plot the tree and then label the
plot, the result is shown in figure \ref{fgini1}.
<>=
progstat <- factor(stagec$pgstat, levels = 0:1, labels = c("No", "Prog"))
cfit <- rpart(progstat ~ age + eet + g2 + grade + gleason + ploidy,
data = stagec, method = 'class')
print(cfit)
par(mar = rep(0.1, 4))
plot(cfit)
text(cfit)
@
\begin{itemize}
\item The creation of a labeled factor variable as the response
improves the labeling of the printout.
\item We have explicitly directed the routine to treat \Co{progstat} as
a categorical variable by asking for \Co{method='class'}.
(Since \Co{progstat} is a factor this would have been the default choice).
Since no optional classification
parameters are specified the routine will use the Gini
rule for splitting, prior probabilities that are proportional
to the observed data frequencies, and 0/1 losses.
\item The child nodes of node $x$ are always $2x$ and $2x+1$, to help
in navigating the printout (compare the printout to figure \ref{fgini1}).
\item Other items in the list are the definition of the split used
to create a node,
the number of subjects at the node,
the loss or error at the node (for this example, with proportional
priors and unit losses this will be the number misclassified),
and the predicted class for the node.
\item * indicates that the node is terminal.
\item Grades 1 and 2 go to the left, grades 3 and 4 go to the right.
The tree is arranged so that the branches with the largest
``average class'' go to the right.
\end{itemize}
\subsection{Variable importance}
The long form of the printout for the stage C data, obtained with the
summary function, contains further information on the surrogates.
The \texttt{cp} option of the summary function instructs it to prune
the printout, but it does not prune the tree.
<>=
@
For each node up to 5 surrogate splits (default) will be printed,
but only those whose utility is greater than the baseline ``go with the
majority'' surrogate.
The first surrogate for the first split is based on the following
table:
<<>>=
temp <- with(stagec, table(cut(grade, c(0, 2.5, 4)),
cut(gleason, c(2, 5.5, 10)),
exclude = NULL))
temp
@
The surrogate sends \Sexpr{sum(diag(temp))} of the \Sexpr{sum(temp)}
observations the correct direction for an agreement of
\Sexpr{round(sum(diag(temp))/sum(temp), 3)}.
The majority rule gets \Sexpr{zz <-max(cfit$frame$n[2:3]); zz} correct, and
the adjusted agreement is (\Sexpr{sum(diag(temp))} - \Sexpr{zz})/
(\Sexpr{sum(temp)} - \Sexpr{zz}).
A variable may appear in the tree many times, either as a primary or a
surrogate variable.
An overall measure of variable importance is the sum of the goodness of split
measures for each split for which it was the primary variable, plus
goodness * (adjusted agreement) for all splits in which it was a surrogate.
In the printout these are scaled to sum to 100 and the rounded values are shown,
omitting any variable whose proportion is less than 1\%.
Imagine two variables which were essentially duplicates of each other;
if we did not count surrogates they would split the importance with neither
showing up as strongly as it should.
\section{Pruning the tree}
\subsection{Definitions}
We have built a complete tree, possibly quite large and/or
complex, and must now decide how much of that model to retain. In
stepwise regression, for instance, this issue is addressed sequentially
and the fit is stopped when the F test fails to achieve some level
$\alpha $.
Let $T_1$, $T_2$,....,$T_k$ be the terminal nodes of a tree T.
Define
\begin{quote}
$|T|$ = number of terminal nodes \\
risk of $T$ = $R(T)$ = $\sum_{i=1}^k P(T_i )R(T_i )$
\end{quote}
In comparison to regression, $|T|$ is analogous to the degrees of freedom
and $R(T)$ to the residual sum of squares.
Now let $\alpha$ be some number between 0 and $\infty$ which measures
the 'cost' of adding another variable to the model; $\alpha$ will be called a
complexity parameter. Let $R(T_0)$ be the risk for the zero split tree.
Define
$$
R_\alpha (T) =R(T) + \alpha |T|
\label{cp1}
$$
to be the cost for the tree, and define $T_\alpha $ to be that
sub tree of the full model which has minimal cost. Obviously $T_0 $
= the full model and $T_\infty $ = the model with no splits at all.
The following results are shown in \cite{Breiman83}.
\begin{enumerate}
\item If $T_1 $ and $T_2 $ are sub trees of $T$ with
$R_\alpha (T_1 )= R_\alpha (T_2 ) $,
then either $T_1 $ is a sub tree of $T_2 $ or $T_2 $ is
a sub tree of $T_1 $; hence either $|T_1| < |T_2|$ or
$|T_2| < |T_1|$.
\item If $\alpha > \beta$ then either $T_\alpha = T_\beta $ or
$T_\alpha $ is a strict sub tree of $T_\beta $.
\item Given some set of numbers $\alpha_1 ,\alpha_2 ,\ldots,\alpha_m $;
both $T_{\alpha_1} ,\ldots,T_{\alpha_m} $ and
$R(T_{\alpha_1} )$, $\ldots$, $R(T_{\alpha_m} )$
can be computed efficiently.
\end{enumerate}
Using the first result, we can uniquely define $T_\alpha$ as the smallest
tree $T$ for which $R_\alpha (T) $ is minimized.
Since any sequence of nested trees based on $T$ has at most
$|T|$ members, result 2 implies that all possible values of $\alpha$
can be grouped into $m$ intervals, $m \le |T| $
\begin{eqnarray*}
I_1 &=& [0, \alpha_1 ] \\
I_2 &=& ( \alpha_1 , \alpha_2 ] \\
\vdots \\
I_m &=& ( \alpha_{m-1} , \infty]
\end{eqnarray*}
where all $\alpha \in I_i $ share the same minimizing sub tree.
\subsection{Cross-validation}
Cross-validation is used to choose a best value for
$\alpha$ by the following steps:
\begin{enumerate}
\item \begin{tabbing}
\= Fit the full model on the data set \\
\> compute $I_1 ,I_2,...,I_m$ \\
\> set \= $\beta_1=0 $ \\
\> \> $\beta_2=\sqrt {\alpha_1 \alpha_2 } $ \\
\> \> $\beta_3=\sqrt {\alpha_2 \alpha_3 } $ \\
\> \> $\vdots $ \\
\> \> $\beta_{m-1} = \sqrt {\alpha_{m-2} \alpha_{m-1} } $ \\
\> \> $\beta_m=\infty$ \\
\> each $\beta_i $ is a `typical value' for its $I_i $ \\
\end{tabbing}
\item Divide the data set into $s$ groups $G_1 ,G_2 ,...,G_s $
each of size $s/n$, and for each group separately:
\begin{itemize}
\item fit a full model on the data set `everyone except $G_i $'
and determine $T_{\beta_1} , T_{\beta_2} ,...,T_{\beta_m} $
for this reduced data set,
\item compute the predicted class for each observation in $G_i$,
under each of the models $T_{\beta_j}$ for $1 \le j \le m$,
\item from this compute the risk for each subject.
\end{itemize}
\item
Sum over the $G_i $ to get an estimate of risk for each
$\beta_j $.
For that $\beta$ (complexity parameter) with smallest risk
compute $T_\beta $ for the full data set, this is
chosen as the best trimmed tree.
\end{enumerate}
In actual practice, we may use instead the 1-SE rule. A plot of $\beta$
versus risk often has an initial sharp drop followed by a relatively flat
plateau and then a slow rise. The choice of $\beta$ among those
models on the
plateau can be essentially random. To avoid this, both an estimate of the
risk and its standard error are computed during the cross-validation. Any
risk within one standard error of the achieved minimum is marked as
being equivalent to the minimum (i.e. considered to be part of the flat
plateau). Then the simplest model, among all those ``tied'' on the
plateau, is chosen.
In the usual definition of cross-validation we would have taken $s=n$ above, i.e.,
each of the $G_i $ would contain exactly one observation, but for
moderate $n$ this is computationally prohibitive. A value of $s = 10$ has
been found to be sufficient, but users can vary this if they wish.
In Monte-Carlo trials, this method of pruning has proven very reliable for
screening out `pure noise' variables in the data set.
\subsection{Example: The Stochastic Digit Recognition Problem}
This example is found in section 2.6 of \cite{Breiman83}, and
used as a running example throughout much of their book.
Consider the segments of an unreliable digital readout
\begin{center}
\begin{picture}(350,160)(-100,0)
\put(27,120){1}
\put(10,115){\line(1,0){40}}
\put(10, 75){\line(1,0){40}}
\put(10, 35){\line(1,0){40}}
\put(10, 35){\line(0,1){80}}
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\put(60, 90){3}
\put(60, 50){6}
\end{picture}
\end{center}
where each light is correct with probability 0.9, e.g., if the true digit is
a 2, the lights 1, 3, 4, 5, and 7 are on with probability 0.9 and lights
2 and 6 are on with probability 0.1. Construct test data where
$Y\in \{0,1,...,9\}$, each with proportion 1/10 and the $X_i$, $i=1,\ldots,7$
are i.i.d. Bernoulli variables with parameter depending on Y.
$X_8 - X_{24} $
are generated as i.i.d Bernoulli $P\{X_i =1\} =.5 $, and are
independent of Y. They correspond to embedding the readout in a larger
rectangle of random lights.
\begin{figure}
\myfig{longintro-dig1}
\caption{Optimally pruned tree for the stochastic digit recognition data}
\label{figdig}
\end{figure}
A sample of size 200 was generated accordingly and the
procedure applied using the Gini index (see 3.2.1) to build the tree.
The R code to compute the simulated data and the fit are shown below.
<>=
set.seed(1953) # An auspicious year
n <- 200
y <- rep(0:9, length = 200)
temp <- c(1,1,1,0,1,1,1,
0,0,1,0,0,1,0,
1,0,1,1,1,0,1,
1,0,1,1,0,1,1,
0,1,1,1,0,1,0,
1,1,0,1,0,1,1,
0,1,0,1,1,1,1,
1,0,1,0,0,1,0,
1,1,1,1,1,1,1,
1,1,1,1,0,1,0)
lights <- matrix(temp, 10, 7, byrow = TRUE) # The true light pattern 0-9
temp1 <- matrix(rbinom(n*7, 1, 0.9), n, 7) # Noisy lights
temp1 <- ifelse(lights[y+1, ] == 1, temp1, 1-temp1)
temp2 <- matrix(rbinom(n*17, 1, 0.5), n, 17) # Random lights
x <- cbind(temp1, temp2)
dfit <- rpart(y ~ x, method='class',
control = rpart.control(xval = 10, minbucket = 2, cp = 0))
printcp(dfit)
fit9 <- prune(dfit, cp = 0.02)
par(mar = rep(0.1, 4))
plot(fit9, branch = 0.3, compress = TRUE)
text(fit9)
@
This table differs from that in section 3.5 of \cite{Breiman83} in
several ways, the last two of which are substantive.
\begin{itemize}
\item The actual values are different, of course, because of different
random number generators in the two runs.
\item The complexity table is printed from the smallest tree (no splits) to the
largest one (28 splits).
We find it easier to compare one tree to another when they start at
the same place.
\item The number of splits is listed, rather than the number of nodes.
The number of nodes is always 1 + the number of splits.
\item For easier reading, the error columns have been scaled so that the
first node has an error of 1. Since in this example the model with no
splits must make 180/200 misclassifications, multiply columns 3-5 by
180 to get a result in terms of absolute error.
(Computations are done on the absolute error scale, and printed on
relative scale).
\item The complexity parameter column has been similarly scaled.
\end{itemize}
Looking at the table, we see that the best tree has 10 terminal
nodes (9 splits), based on cross-validation.
This sub tree is extracted with a call to \Co{prune} and saved in \Co{fit9}.
The pruned tree is shown in figure \ref{figdig}.
Two options have been used in the plot. The \Co{compress} option
tries to narrow the printout by vertically overlapping portions
of the plot.
The \Co{branch} option controls the shape of the branches that
connect a node to its children.
The largest tree, with 35 terminal nodes, correctly classifies 170/200 = 85\%
of the observations, but uses several of the random predictors in
doing so and seriously over fits the data.
If the number of observations per terminal node (minbucket)
had been set to 1 instead of
2, then every observation would be classified correctly in the final model,
many in terminal nodes of size 1.
\section{Missing data}
\subsection{Choosing the split}
Missing values are one of the curses of statistical models and analysis.
Most procedures deal with them by refusing to deal with them --
incomplete observations are tossed out. \texttt{Rpart} is somewhat more
ambitious. Any observation with values for the dependent variable and
at least one independent variable will participate in the modeling.
The quantity to be maximized is still
$$
\Delta I = p(A)I(A)-p(A_L )I(A_L ) - p(A_R ) I(A_R)
$$
The leading term is the same for all variables and splits irrespective of
missing data, but the right two terms are somewhat modified. Firstly, the
impurity indices $I(A_R )$ and $I (A_L )$ are calculated only over
the observations which are not missing a particular predictor. Secondly,
the two probabilities $p(A_L )$ and $p(A_R )$ are also calculated
only over the relevant observations, but they are then adjusted so that
they sum to $p(A)$. This entails some extra bookkeeping as the tree is
built, but ensures that the terminal node probabilities sum to 1.
In the extreme case of a variable for which only 2 observations are
non-missing, the impurity of the two sons will both be zero when splitting
on that variable. Hence $\Delta I $ will be maximal, and this `almost all
missing' coordinate is guaranteed to be chosen as best; the method is
certainly flawed in this extreme case. It is difficult to say whether this
bias toward missing coordinates carries through to the non-extreme cases,
however, since a more complete variable also affords for itself more possible
values at which to split.
\subsection{Surrogate variables}
Once a splitting variable and a split point for it have been decided,
what {\em is}
to be done with observations missing that variable? One approach is to
estimate the missing datum using the other independent variables; {\rpart}
uses a variation of this to define
{\em surrogate} variables.
As an example, assume that the split (age $<$40, age $\ge $40) has
been chosen. The surrogate variables are found by re-applying the
partitioning algorithm (without recursion) to predict the two categories
`age $<$40' vs. `age $\ge $40' using the other independent variables.
%$$
%\begin{tabular}{l|r|r|}
% & & \multicolumn{2}{c}{Age $<$ 40} \\
% & & True & False \\ \hline
% Other X $<$ cutpoint & True & True Positive & False Positive \\ \cline{3-4}
% & False & False Negative & True Negative \\ \hline
%\end{tabular}
%$$
For each predictor an optimal split point and a misclassification error
are computed. (Losses and priors do not enter in --- none are defined for
the age groups --- so the risk is simply \#misclassified / n.) Also evaluated
is the blind rule `go with the majority' which has misclassification
error $\min(p, 1-p)$ where
\begin{center}
$p$ = (\# in A with age $<$ 40) / $n_A $.
\end{center}
The surrogates are ranked, and any variables which do no better than the
blind rule are discarded from the list.
Assume that the majority of subjects have age $\le$ 40 and that there is
another variable $x$ which is uncorrelated to
age; however, the subject with the largest value of $x$ is also over
40 years of age. Then the surrogate variable
$x < \max$ versus $x \ge \max$ will have one less error that the
blind rule,
sending 1 subject to the right and $n-1$ to the left.
A continuous variable that is completely unrelated to age has probability
$1-p^2$ of generating such a
trim-one-end surrogate by chance alone. For this reason
the {\rpart} routines impose one more constraint during the construction
of the surrogates: a candidate split must send at least 2 observations to
the left and at least 2 to the right.
Any observation which is missing the split variable is then classified using
the first surrogate variable, or if missing that, the second surrogate
is used, and etc. If an observation is missing all the surrogates the blind
rule is used. Other strategies for these `missing everything' observations
can be convincingly argued, but there should be few or no observations
of this type (we hope).
\subsection{Example: Stage C prostate cancer (cont.)}
Let us return to the stage C prostate cancer data of the
earlier example.
For a more detailed listing of the rpart object, we use the \Co{summary}
function. It includes the information from the CP table (not repeated below),
plus information about each node. It is easy to print a
sub tree based on a different cp value using the \Co{cp} option.
Any value between 0.0555 and 0.1049 would produce the same
result as is listed below, that is, the tree with 3 splits.
Because the printout is long, the \Co{file} option of \Co{summary.rpart}
is often useful.
<<>>=
printcp(cfit)
summary(cfit, cp = 0.06)
@
\begin{itemize}
\item There are 54 progressions (class 1) and 94 non-progressions, so
the first node has an expected loss of $54/146 \approx 0.37$.
(The computation is this simple only for the default priors and losses).
\item Grades 1 and 2 go to the left, grades 3 and 4 to the right.
The tree is arranged so that the ``more severe'' nodes go to the
right.
\item The improvement is $n$ times the change in impurity index.
In this instance, the largest improvement is for the variable \Co{grade},
with an improvement of 10.36.
The next best choice is Gleason score, with an improvement of 8.4.
The actual values of the improvement are not so important, but their
relative size gives an indication of the comparative utility of the
variables.
\item Ploidy is a categorical variable, with values of
diploid, tetraploid, and aneuploid, in that order.
(To check the order, type \Co{table(stagec\$ploidy)}).
All three possible splits were attempted:
aneuploid+diploid vs. tetraploid, aneuploid+tetraploid vs. diploid,
and aneuploid vs. diploid + tetraploid.
The best split sends diploid to the right and the others to the left.
\item The 2 by 2 table of diploid/non-diploid vs grade= 1-2/3-4 has
64\% of the observations on the diagonal. \\
$$
\begin{tabular}{r|rr}
& 1-2 & 3-4 \\ \hline
diploid & 38 & 29 \\
tetraploid & 22 & 46 \\
aneuploid & 1 & 10
\end{tabular}
$$
\item For node 3, the primary split variable is missing on 6 subjects.
All 6 are split based on the first surrogate, ploidy. Diploid and
aneuploid tumors are sent to the left, tetraploid to the right.
As a surrogate,
eet was no better than 45/85 (go with the majority), and was not retained.
$$
\begin{tabular}{l|c|c|}
& g2 $<$ 13.2 & g2 $>$ 13.2 \\ \hline
Diploid/aneuploid & 71 & 1 \\
Tetraploid & 3 & 64 \\
\end{tabular}
$$
\end{itemize}
\section{Further options}
\subsection{Program options}
The central fitting function is {\rpart}, whose main arguments are
\begin{itemize}
\item \Co{formula}: the model formula, as in \Co{lm} and other R
model fitting functions. The right hand side may contain both
continuous and categorical (factor) terms. If the outcome $y$ has more
than two levels, then categorical predictors must be fit by exhaustive
enumeration, which can take a very long time.
\item \Co{data, weights, subset}: as for other R models.
\item \Co{method}: the type of splitting rule to use. Options
at this point are classification, anova, Poisson, and exponential.
\item \Co{parms}: a list of method specific optional parameters.
For classification, the
list can contain any of: the vector of prior probabilities
(component \Co{prior}), the loss matrix (component \Co{loss})
or the splitting index (component \Co{split}).
The priors must be positive and sum to 1. The loss matrix must have
zeros on the diagonal and positive off-diagonal elements.
The splitting index can be \Co{"gini"} or \Co{"information"}.
\item \Co{na.action}: the action for missing values. The default action
for rpart is \Co{na.rpart}, this default is not overridden by the
\Co{options(na.action)} global option. The default action removes only
those rows for which either the response $y$ or \emph{all} of the predictors
are missing.
This ability to retain partially missing observations is perhaps the single
most useful feature of {\rpart} models.
\item \Co{control}: a list of control parameters, usually the result of
the \Co{rpart.control} function. The list must contain
\begin{itemize}
\item \Co{minsplit}: The minimum number of observations in a node for which
the routine will even try to compute a split. The default is 20.
This parameter can save computation time, since smaller nodes
are almost always pruned away by cross-validation.
\item \Co{minbucket}: The minimum number of observations in a terminal node.
This defaults to \Co{minsplit/3}.
\item \Co{maxcompete}: It is often useful in the printout to see not only
the variable that gave the best split at a node, but also the second, third,
etc best. This parameter controls the number that will be printed. It has
no effect on computational time, and a small effect on the amount of
memory used. The default is 4.
\item \Co{xval}: The number of cross-validations to be done. Usually set to
zero during exploratory phases of the analysis. A value of 10, for instance,
increases the compute time to 11-fold over a value of 0.
\item \Co{maxsurrogate}: The maximum number of surrogate variables to retain
at each node. (No surrogate that does worse than ``go with the majority''
is printed or used). Setting this to zero will cut the computation time in
half, and set \Co{usesurrogate} to zero. The default is 5.
Surrogates give different information than competitor splits. The competitor
list asks ``which other splits would have as many correct classifications'',
surrogates ask ``which other splits would classify the same subjects in the
same way'', which is a harsher criteria.
\item \Co{usesurrogate}: A value of usesurrogate=2, the default, splits
subjects in the way described previously. This is similar to CART.
If the value is 0, then a subject who is missing the primary split variable
does not progress further down the tree. A value of 1 is intermediate:
all surrogate variables except ``go with the majority'' are used to
send a case further down the tree.
\item \Co{cp}: The threshold complexity parameter.
\end{itemize}
\end{itemize}
The complexity parameter \Co{cp} is, like \Co{minsplit}, an advisory parameter,
but is considerably more useful. It is specified according to the
formula
$$
R_{cp}(T) \equiv R(T) + cp* |T| * R(T_1)
$$
where $T_1$ is the tree with no splits, $|T|$ is the
number of splits for a tree, and $R$ is the risk.
This scaled version is much more user friendly than the original
CART formula (\ref{cp1}) since it is unit less.
A value of \Co{cp = 1} will always result in a tree with no splits.
For regression models (see next section) the scaled cp has a very
direct interpretation: if any split does not increase the overall
${\rm R}^2$ of the model by at least $cp$
(where ${\rm R}^2$ is the usual linear-models definition)
then that split is decreed to be, a priori, not worth pursuing.
The program does not split said branch any further, and
saves considerable computational effort.
The default value of .01 has been reasonably successful at
`pre-pruning' trees so that the cross-validation step need only
remove 1 or 2 layers, but it sometimes over prunes,
particularly for large data sets.
\subsection{Example: Consumer Report Auto Data}
A second example using the \Co{class} method demonstrates the outcome
for a response with multiple ($>2$) categories. We also explore
the difference between Gini and information splitting rules.
The dataset cu.summary contains a collection of variables from the April,
1990 Consumer Reports summary on 117 cars. For our purposes, car
Reliability will be treated as the response. The variables are:
$$
\begin{tabular}{ll}
Reliability & an ordered factor (contains NAs): \\
& Much worse $<$ worse $<$ average $<$ better
$<$ Much Better \\
Price & numeric: list price in dollars, with standard equipment\\
Country & factor: country where car manufactured \\
Mileage & numeric: gas mileage in miles/gallon, contains NAs \\
Type & factor: Small, Sporty, Compact, Medium, Large, Van \\
\end{tabular}
$$
In the analysis we are treating reliability as an unordered outcome.
Nodes potentially can be classified as Much worse, worse, average,
better, or Much better, though there are none that are labeled as
just ``better''. The 32 cars with missing response (listed as NA)
were not used in the analysis.
Two fits are made, one using the Gini index and the other the
information index.
\begin{figure}[tb]
\myfig{longintro-cars}
\caption{Displays the rpart-based model relating automobile
Reliability to car type, price, and country of origin. The figure
on the left uses the \texttt{gini} splitting
index and the figure on the right uses the \texttt{information} splitting
index.}
\label{ginifig5}
\end{figure}
<>=
fit1 <- rpart(Reliability ~ Price + Country + Mileage + Type,
data = cu.summary, parms = list(split = 'gini'))
fit2 <- rpart(Reliability ~ Price + Country + Mileage + Type,
data = cu.summary, parms = list(split = 'information'))
par(mfrow = c(1,2), mar = rep(0.1, 4))
plot(fit1, margin = 0.05); text(fit1, use.n = TRUE, cex = 0.8)
plot(fit2, margin = 0.05); text(fit2, use.n = TRUE, cex = 0.8)
@
Due to the wide labels at the bottom, we had to increase the figure space
slightly and decrease the character size in order to avoid truncation at the
left and right edges.
Details for the first two splits in the Gini tree are
<<>>=
summary(fit1, cp = 0.06)
@
And for the information splitting the first split is
<<>>=
fit3 <- rpart(Reliability ~ Price + Country + Mileage + Type,
data=cu.summary, parms=list(split='information'),
maxdepth=2)
summary(fit3)
@
The first 3 countries (Brazil, England, France) had only one or two
cars in the listing, all of which were missing the reliability
variable. There are no entries for these countries in the first node,
leading to the $-$ symbol for the rule.
The information measure has larger ``improvements'', consistent with
the difference in scaling between the information and Gini criteria
shown in figure 2, but the relative merits of different splits are
fairly stable.
The two rules do not choose the same primary split at node 2.
The data at this point are
\begin{center}
\begin{tabular}{rcccccc}
&Compact & Large & Medium &Small & Sporty & Van \\
Much worse & 2 & 2 & 4 & 2 & 7 & 1 \\
worse & 5 & 0 & 4 & 3 & 0 & 0 \\
average & 3 & 5 & 8 & 2 & 2 & 3 \\
better & 2 & 0 & 0 & 3 & 0 & 0 \\
Much better & 0 & 0 & 0 & 0 & 0 & 0
\end{tabular}
\end{center}
Since there are 6 different categories, all $2^5 = 32$ different
combinations were explored, and as it turns out there are several
with a nearly identical improvement. The Gini and information
criteria make different ``random'' choices from this set of near
ties. For the Gini index, \emph{Sporty vs others} and
\emph{Compact/Small vs others} have improvements of 3.19 and 3.12,
respectively.
For the information index, the improvements are
6.21 versus 9.28.
Thus the Gini index picks the first rule and information the second.
Interestingly, the two splitting criteria arrive at exactly the
same final nodes, for the full tree, although by different paths.
(Compare the class counts of the terminal nodes).
We have said that for a categorical predictor with $m$ levels,
all $2^{m-1}$ different possible splits are tested..
When there are a large number of categories for the predictor,
the computational burden of evaluating all of these subsets can become
large.
For instance, the call \Co{rpart(Reliability ~ ., data=car90)}
does not return for a \emph{long,} long time: one of the predictors
in that data set is an unordered factor with 30 levels!
Luckily, for any ordered outcome there is a computational
shortcut that allows the program to find the best split using
only $m-1$ comparisons. This includes the classification method
when there are only two categories, along with the
anova and Poisson methods to be introduced later.
\subsection{Example: Kyphosis data}
A third \Co{class} method example explores
the parameters \Co{prior} and \Co{loss}.
The dataset kyphosis has 81 rows representing data on 81 children who
have had corrective spinal surgery. The variables are:
$$
\begin{tabular}{ll}
Kyphosis & factor: postoperative deformity is present/absent \\
Age & numeric: age of child in months \\
Number & numeric: number of vertebrae involved in operation \\
Start & numeric: beginning of the range of vertebrae involved \\
\end{tabular}
$$
\begin{figure}
\myfig{longintro-kyphos}
\caption{Displays the rpart-based models for the
presence/absence of kyphosis. The figure
on the left uses the default {prior} (0.79,0.21) and {loss}; the
middle figure uses the user-defined {prior} (0.65,0.35) and default
{loss}; and the third figure uses the default {prior} and the
user-defined {loss} $L(1,2)=3,\, L(2,1)=2$).}
\label{kyphfig}
\end{figure}
<>=
lmat <- matrix(c(0,3, 4,0), nrow = 2, ncol = 2, byrow = FALSE)
fit1 <- rpart(Kyphosis ~ Age + Number + Start, data = kyphosis)
fit2 <- rpart(Kyphosis ~ Age + Number + Start, data = kyphosis,
parms = list(prior = c(0.65, 0.35)))
fit3 <- rpart(Kyphosis ~ Age + Number + Start, data = kyphosis,
parms = list(loss = lmat))
par(mfrow = c(1, 3), mar = rep(0.1, 4))
plot(fit1); text(fit1, use.n = TRUE, all = TRUE, cex = 0.8)
plot(fit2); text(fit2, use.n = TRUE, all = TRUE, cex = 0.8)
plot(fit3); text(fit3, use.n = TRUE, all = TRUE, cex = 0.8)
@
This example shows how even the initial split changes depending on the
prior and loss that are specified. The first and third fits have the
same initial split (Start $< 8.5$), but the improvement differs. The
second fit splits Start at 12.5 which moves 46 people to the left
instead of 62.
Looking at the leftmost tree, we see that the sequence of
splits on the left hand branch yields only a single node classified
as {\em present}. For any loss greater than 4 to 3, the routine will
instead classify this node as {\em absent}, and the entire left side of
the tree collapses, as seen in the right hand figure.
This is not unusual --- the most common effect of alternate loss
matrices is to change the amount of pruning in the tree, more pruning in
some branches and less in others, rather than to change the
choice of splits.
The first node from the default tree is
\begin{verbatim}
Node number 1: 81 observations, complexity param=0.1765
predicted class= absent expected loss= 0.2099
class counts: 64 17
probabilities: 0.7901 0.2099
left son=2 (62 obs) right son=3 (19 obs)
Primary splits:
Start < 8.5 to the right, improve=6.762, (0 missing)
Number < 5.5 to the left, improve=2.867, (0 missing)
Age < 39.5 to the left, improve=2.250, (0 missing)
Surrogate splits:
Number < 6.5 to the left, agree=0.8025, (0 split)
\end{verbatim}
The fit using the prior (0.65,0.35) is
\begin{Verbatim}
Node number 1: 81 observations, complexity param=0.302
predicted class= absent expected loss= 0.35
class counts: 64 17
probabilities: 0.65 0.35
left son=2 (46 obs) right son=3 (35 obs)
Primary splits:
Start < 12.5 to the right, improve=10.900, (0 missing)
Number < 4.5 to the left, improve= 5.087, (0 missing)
Age < 39.5 to the left, improve= 4.635, (0 missing)
Surrogate splits:
Number < 3.5 to the left, agree=0.6667, (0 split)
\end{Verbatim}
And first split under 4/3 losses is
\begin{Verbatim}
Node number 1: 81 observations, complexity param=0.01961
predicted class= absent expected loss= 0.6296
class counts: 64 17
probabilities: 0.7901 0.2099
left son=2 (62 obs) right son=3 (19 obs)
Primary splits:
Start < 8.5 to the right, improve=5.077, (0 missing)
Number < 5.5 to the left, improve=2.165, (0 missing)
Age < 39.5 to the left, improve=1.535, (0 missing)
Surrogate splits:
Number < 6.5 to the left, agree=0.8025, (0 split)
\end{Verbatim}
\section{Regression}
\label{method}
\subsection{Definition}
Up to this point the classification problem has been used to define and
motivate our formulae. However, the partitioning procedure is quite
general and can be extended by specifying 5 ``ingredients'':
\begin{itemize}
\item A splitting criterion, which is used to decide which variable
gives the best split.
For classification this was either the Gini or log-likelihood function.
In the \Co{anova} method the splitting criteria is
$SS_T - (SS_L + SS_R)$, where
$SS_T = \sum(y_i - \bar y)^2$
is the sum of squares for the node, and $SS_R$, $SS_L$ are the
sums of squares for the right and left son, respectively.
This is equivalent to choosing the split to maximize the between-groups
sum-of-squares in a simple analysis of variance. This rule is
identical to the regression option for {\tree}.
\item A summary statistic or vector, which is used to describe a
node. The first element of the vector is considered to be the
fitted value. For the anova method this is the mean of the node;
for classification the response is the predicted class followed by
the vector of class probabilities.
\item The error of a node. This will be the variance of $y$ for anova,
and the predicted loss for classification.
\item The prediction error for a new observation, assigned to the node.
For anova this is $(y_{new} - \bar y)$.
\item Any necessary initialization.
\end{itemize}
The \Co{anova} method leads to regression trees; it is the
default method if $y$ a simple numeric vector, i.e., not a
factor, matrix, or survival object.
\subsection{Example: Consumer Report car data}
The dataset \Co{car90} contains a collection of variables from the
April, 1990 Consumer Reports; it has 34 variables on 111 cars.
We've excluded 3 variables: tire size and model name because they are factors
with a very large number of levels whose printout does not fit well in
this report's page size, and rim size because it is too good a predictor of
price and leads to a less interesting illustration. (Tiny cars are cheaper
and have small rims.)
<<>>=
cars <- car90[, -match(c("Rim", "Tires", "Model2"), names(car90))]
carfit <- rpart(Price/1000 ~ ., data=cars)
carfit
printcp(carfit)
@
<>=
temp <- carfit$cptable
@
\begin{itemize}
\item The relative error is $1-{\rm R}^2$, similar to linear regression.
The xerror is related to the PRESS statistic.
The first split appears to improve the fit the most.
The last split adds little improvement to the apparent error,
and increases the cross-validated error.
\item The 1-SE rule would choose a tree with 3 splits.
\item This is a case where the default cp value of .01 may
have over pruned the tree, since the cross-validated error
is barely at a minimum. A rerun with the cp threshold at .001
gave little change, however.
\item For any CP value between Sexpr{round(temp[3,1],2)} and
\Sexpr{round(temp[2,1],2)} the best
model has one split;
for any CP value between \Sexpr{round(temp[4,1],2)} and
\Sexpr{round(temp[3,1],2)} the best model is with
2 splits; and so on.
\end{itemize}
The \Co{print} and \Co{summary} commands also recognizes the \Co{cp} option, which
allows the user to look at only the top few splits.
<<>>=
summary(carfit, cp = 0.1)
@
The first split on displacement partitions the
\Sexpr{carfit$frame$n[1]} observations into groups of
\Sexpr{carfit$frame$n[2]} and \Sexpr{carfit$frame$n[3]}
(nodes 2 and 3) with mean prices of
\Sexpr{round(carfit$frame$yval[2])} and
\Sexpr{round(carfit$frame$yval[3])}
\begin{itemize}
\item The improvement listed is the percent change in SS for this split,
i.e., $1 - (SS_{right} + SS_{left})/SS_{parent}$, which is the gain in
$R^2$ for the fit.
\item The data set has displacement of the engine in both cubic inches
(Disp) and liters (Disp2). The second is a perfect surrogate split for
the first, obviously.
\item The weight and displacement are very closely related,
as shown by the surrogate split agreement of 91\%.
\item Not all the countries are represented in node 3, e.g., there are no
larger cars from Brazil.
This is indicated by a \Co{-} in the list of split directions.
\end{itemize}
\begin{figure}
\myfig{longintro-anova2}
\caption{Both plots were obtained using the function
\Co{rsq.rpart(fit3)}. The figure on the left shows that the first split
offers the most information. The figure on the right suggests that
the tree should be pruned to include only 1 or 2 splits.}
\label{anovafig2}
\end{figure}
<>=
par(mfrow=c(1,2))
rsq.rpart(carfit)
par(mfrow=c(1,1))
@
Other plots can be used to help determine the best cp value for this
model. The function \Co{rsq.rpart} plots the jackknifed error versus
the number of splits. Of interest is the smallest error, but any
number of splits within the ``error bars'' (1-SE rule) are considered
a reasonable number of splits (in this case, 1 or 2 splits seem to be
sufficient). As is often true with modeling, simpler is often
better. Another useful plot is the ${\rm R}^2$ versus number of
splits. The (1 - apparent error) and (1 - relative error) show how much
is gained with additional splits. This plot highlights the
differences between the ${\rm R}^2$ values.
\begin{figure}
\myfig{longintro-anova3}
\caption{This plot shows the (observed-expected) cost of cars
versus the predicted cost of cars based on the nodes/leaves in which the cars
landed. There appears to be more variability in node 7 than in some of
the other leaves.}
\label{anovafig3}
\end{figure}
Finally, it is possible to look at the residuals from this model, just
as with a regular linear regression fit, as shown in
figure \ref{anovafig3} produced by the following.
<>=
plot(predict(carfit), jitter(resid(carfit)))
temp <- carfit$frame[carfit$frame$var == '',]
axis(3, at = temp$yval, as.character(row.names(temp)))
mtext('leaf number', side = 3, line = 3)
abline(h = 0, lty = 2)
@
\subsection{Example: Stage C data (\Co{anova} method)}
The stage C prostate cancer data of the earlier section can also
be fit using the anova method, by treating the status variable
as though it were continuous.
<<>>=
cfit2 <- rpart(pgstat ~ age + eet + g2 + grade + gleason + ploidy,
data = stagec)
printcp(cfit2)
print(cfit2, cp = 0.03)
@
If this tree is compared to the earlier results, we see that it
has chosen exactly the same variables and split points as before.
The only addition is further splitting of node 2, the upper left
``No'' of figure \ref{fgini1}. This is no accident, for the two
class case the Gini splitting rule reduces to $2p(1-p)$, which is
the variance of a node.
The two methods differ in their evaluation and pruning, however.
Note that nodes 4 and 5, the two children of node 2, contain
2/40 and 7/21 progressions, respectively. For classification
purposes both nodes have the same predicted value (No) and the
split will be discarded since the error (\# of misclassifications)
with and without the split is identical.
In the regression context the two predicted values of .05 and .33
{\em are} different --- the split has identified a nearly pure
subgroup of significant size.
This setup is known as \emph{odds regression}, and may be a more
sensible way to evaluate a split when the emphasis of the model
is on understanding/explanation rather than on prediction error
per se. Extension of this rule to the multiple class problem
is appealing, but has not yet been implemented in {\rpart}.
\section{Poisson regression}
\subsection{Definition}
The Poisson splitting method attempts to extend {{\rpart}} models to
event rate data.
The model in this case is
$$
\lambda = f(x)
$$
where $\lambda$ is an event rate and $x$ is some set of predictors. As an
example consider hip fracture rates. For each county in the United States
we can obtain
\begin{itemize}
\item number of fractures in patients age 65 or greater (from Medicare files)
\item population of the county (US census data)
\item potential predictors such as
\begin{itemize}
\item socio-economic indicators
\item number of days below freezing
\item ethnic mix
\item physicians/1000 population
\item etc.
\end{itemize}
\end{itemize}
Such data would usually be approached by using Poisson regression; can we
find a tree based analogue?
In adding criteria for rates regression to this ensemble, the guiding
principle was the following: the between groups sum-of-squares is not a
very robust measure, yet tree based regression works fairly well for this
data.
So do the simplest (statistically valid) thing possible.
Let $c_i$ be the observed event count for observation $i$, $t_i$ be the
observation time, and $x_{ij}, j=1,\ldots,p$ be the predictors.
The $y$ variable for the program will be a 2 column matrix.
Splitting criterion: The likelihood ratio test for two Poisson groups
$$
D_{\hbox{parent}} - \left( D_{\hbox{left son}} + D_{\hbox{right son}}
\right )
$$
Summary statistics: The observed event rate and the number of events.
\begin{eqnarray*}
\hat\lambda &=& \frac{\hbox{\# events}}{\hbox{total time}}
= \frac{\sum c_i}{\sum t_i} \\
\end{eqnarray*}
Error of a node: The within node deviance.
$$
D = \sum \left[ c_i \log \left(\frac{c_i}{\hat\lambda t_i} \right)
- (c_i - \hat\lambda t_i) \right]
$$
Prediction error: The deviance contribution for a new observation, using
$\hat \lambda$ of the node as the predicted rate.
\subsection{Improving the method}
There is a problem with the criterion just proposed, however:
cross-validation of a model often produces an infinite value for the
deviance. The simplest case where this occurs is easy to understand.
Assume that some terminal node of the tree has 20 subjects, but only 1 of
the 20 has experienced any events. The cross-validated error (deviance)
estimate for that node will have one subset --- the one where the subject
with an event is left out --- which has $\hat\lambda=0$.
When we use the prediction for the 10\% of subjects who were set aside,
the deviance contribution of the subject with an event is
$$
\ldots + c_i \log(c_i / 0) + \ldots
$$
which is infinite since $c_i >0$. The problem is that when $\hat\lambda=0$
the occurrence of an event is infinitely improbable, and, using the deviance
measure, the corresponding model is then infinitely bad.
One might expect this phenomenon to be fairly rare, but unfortunately it is
not so. One given of tree-based modeling is that a right-sized model is
arrived at by purposely over-fitting the data and then pruning back the
branches. A program that aborts due to a numeric exception during the first
stage is uninformative to say the least.
Of more concern is that this edge effect does not seem to be limited to
the pathological case detailed above. Any near approach to the boundary value
$\lambda=0$ leads to large values of the deviance, and the procedure
tends to discourage any final node with a small number of events.
An ad hoc solution is to use the revised estimate
$$
\hat{\hat \lambda} = \max \left(\hat\lambda, \frac{k}{\sum t_i} \right)
$$
where $k$ is 1/2 or 1/6.
That is, pure nodes are given a partial event.
(This is similar to the starting estimates used in
the GLM program for a Poisson regression.) This is unsatisfying, however, and
we propose instead using a shrinkage estimate.
Assume that the true rates $\lambda_j$ for the leaves of the tree are
random values from a Gamma$(\mu, \sigma)$ distribution. Set
$\mu$ to the observed overall event rate $\sum c_i / \sum t_i$, and
let the user choose as a prior the coefficient of variation
$k =\sigma / \mu$. A value of $k=0$ represents extreme pessimism
(``the leaf nodes will all give the same result''), whereas $k=\infty$
represents
extreme optimism. The Bayes estimate of the event rate for a node works out
to be
$$
\hat \lambda_k = \frac{\alpha + \sum c_i}{\beta + \sum t_i},
$$
where $\alpha= 1/k^2$ and $\beta = \alpha/ \hat\lambda$.
This estimate is scale invariant, has a simple interpretation, and
shrinks least those nodes with a large amount of information. In practice,
a value of $k=10$ does essentially no shrinkage.
For \Co{method='poisson'}, the optional parameters list is the single
number $k$, with a default value of 1.
\subsection{Example: solder.balance data}
The solder.balance data frame, as explained in the R help file, is a
dataset with 900 observations which are the results of an
experiment varying 5 factors relevant to the wave-soldering procedure
for mounting components on printed circuit boards. The full version of the data (unbalanced) is available in the \Co{survival} package. The response
variable, skips, is a count of how many solder skips appeared to a
visual inspection. The other variables are listed below:
$$
\begin{tabular}{ll}
Opening & factor: amount of clearance around
the mounting pad (S $<$ M $<$ L) \\
Solder & factor: amount of solder used (Thin $<$ Thick) \\
Mask & factor: Type of solder mask used (5 possible) \\
PadType & factor: Mounting pad used (10 possible) \\
Panel & factor: panel (1, 2 or 3) on board being counted \\
\end{tabular}
$$
In this call, the \Co{rpart.control} options are modified:
\Co{maxcompete = 2} means that only 2 other competing splits are
listed (default is 4); \Co{cp = .05} means that a smaller
tree will be built initially (default is .01).
The $y$ variable for Poisson partitioning may be a two column
matrix containing the observation time in column 1 and the
number of events in column 2, or it may be a vector of event
counts alone.
<<>>=
sfit <- rpart(skips ~ Opening + Solder + Mask + PadType + Panel,
data = solder.balance, method = 'poisson',
control = rpart.control(cp = 0.05, maxcompete = 2))
sfit
@
\begin{itemize}
\item The response value is the expected event rate
(with a time variable), or in this case the expected number of skips.
The values are shrunk towards the global estimate of 5.53 skips/observation.
\item The deviance is the same as the null deviance (sometimes called the
residual deviance) that you'd get when calculating a Poisson glm model
for the given subset of data.
\end{itemize}
<<>>=
summary(sfit, cp = 0.1)
@
\begin{itemize}
\item The improvement is ${\rm Deviance_{parent} - (Deviance_{left} +
Deviance_{right})}$, which is the likelihood ratio test
for comparing two Poisson samples.
\item The cross-validated error has been found to be
overly pessimistic when describing how much the error
is improved by each split. This is likely an effect of the
boundary effect mentioned earlier, but more research is needed.
\item The variation xstd is not as useful, given the bias of xerror.
\end{itemize}
\begin{figure}
\myfig{longintro-poisson1}
\caption{The first figure shows the solder.balance data, fit with the
\Co{poisson} method, using a \Co{cp} value of 0.05. The second figure
shows the same fit, but with a \Co{cp} value of 0.10. The function
\Co{prune.rpart} was used to produce the smaller tree.}
\label{poissonfig1}
\end{figure}
<>=
par(mar = rep(0.1, 4))
plot(sfit)
text(sfit, use.n = TRUE, min = 3)
fit.prune <- prune(sfit, cp = 0.10)
plot(fit.prune)
text(fit.prune, use.n = TRUE, min = 2)
@
The \Co{use.n = TRUE} option specifies that number of events / total N should be
listed along with the predicted rate (number of events/person-years).
The function \Co{prune} trims the tree \Co{fit} to the \Co{cp} value
$0.10$. The same tree could have been created by specifying
\Co{cp = 0.10} in the original call to {\rpart}.
\subsection{Example: Stage C Prostate cancer, survival method}
One special case of the Poisson model is of particular interest
for medical consulting (such as the authors do).
Assume that we have survival data, i.e., each subject has either 0 or 1 event.
Further, assume that the time values have been pre-scaled so as to
fit an exponential model. That is, stretch the time axis so that a
Kaplan-Meier plot of the data will be a straight line when plotted on
the logarithmic scale.
An approximate way to do this is
<<>>=
require(survival)
temp <- coxph(Surv(pgtime, pgstat) ~ 1, stagec)
newtime <- predict(temp, type = 'expected')
@
and then do the analysis using the \Co{newtime} variable.
(This replaces each time value by $\Lambda(t)$, where $\Lambda$ is the
cumulative hazard function).
A slightly more sophisticated version of this which we
will call \emph{exponential scaling}
gives a straight line curve for log(survival) under a
parametric exponential model.
The only difference from the approximate scaling above is that
a subject who is censored between observed death times
will receive ``credit'' for the intervening interval, i.e.,
we assume the baseline hazard to be linear between observed deaths.
If the data is pre-scaled in this way, then the
Poisson model above is equivalent to the \emph{local full likelihood}
tree model of LeBlanc and Crowley \cite{Leblanc92}.
They show that this model is more efficient than the earlier suggestion
of Therneau et al. \cite{Therneau90} to use the martingale residuals
from a Cox model as input to a regression tree (anova method).
Exponential scaling or method='exp' is
the default if $y$ is a \Co{Surv} object.
Let us again return to the stage C cancer example.
Besides
the variables explained previously, we will use pgtime, which is time
to tumor progression.
<>=
require(survival)
pfit <- rpart(Surv(pgtime, pgstat) ~ age + eet + g2 + grade +
gleason + ploidy, data = stagec)
print(pfit)
pfit2 <- prune(pfit, cp = 0.016)
par(mar = rep(0.2, 4))
plot(pfit2, uniform = TRUE, branch = 0.4, compress = TRUE)
text(pfit2, use.n = TRUE)
@
Note that the primary split on grade is the same as when
status was used as a dichotomous endpoint,
but that the splits thereafter differ.
Suppose that we wish to simplify this tree, so that only
four splits remain.
Looking at the table of complexity parameters, we see that
\Co{prune(fit, cp = 0.016)} would give the desired result,
which is shown in figure \ref{expfig3}.
From the figure node 4 (leftmost terminal) has only 1 event
for 33 subjects, a relative death rate of .133 times the
overall rate, and is defined by grade = 1--2 and
g2 $<$ 11.36.
\begin{figure}
\myfig{longintro-exp3}
\caption{Survival fit to the stage C prostate data.}
\label{expfig3}
\end{figure}
For a final summary of the model, it can be helpful to plot the
probability of survival based on the final bins in which the subjects
landed. To create new variables based on the rpart groupings,
using the \Co{where} component of the fit, as
shown in figure \ref{expfig4}.
We'll further prune the tree down to four nodes by removing the
split at node 6.
\begin{figure}
\myfig{longintro-exp4}
\caption{Survival plot based on snipped rpart object. The
probability of tumor progression is greatest in node 8, which has
patients who are older and have a higher initial tumor grade. }
\label{expfig4}
\end{figure}
<>=
temp <- snip.rpart(pfit2, 6)
km <- survfit(Surv(pgtime, pgstat) ~ temp$where, stagec)
plot(km, lty = 1:4, mark.time = FALSE,
xlab = "Years", ylab = "Progression")
legend(10, 0.3, paste('node', c(4,5,6,7)), lty = 1:4)
@
\subsection{Open issues}
The default value of the shrinkage parameter $k$ is 1. This corresponds
to prior coefficient of variation of 1 for the estimated $\lambda_j$.
We have not nearly enough experience to decide if this is a good value.
(It does stop the $\log(0)$ message though).
Cross-validation does not work very well. The procedure gives very
conservative results, and quite often declares the no-split tree to
be the best. This may be another artifact of the edge effect.
\section{Plotting options}
\label{plotsect}
This section examines the various options that are available when
plotting an rpart object. For simplicity, the same model (data from
Example 1) will be used throughout.
You have doubtless already noticed the use of \texttt{par(mar =)}
in the examples.
The plot function for rpart uses the general \texttt{plot} function
to set up the plot region. By default, this leaves space for
axes, legends or titles on the bottom, left, and top.
Room for axes is not needed in general for rpart plots, and
for this report we also do not have top titles.
For the small plots in this report it was important to use all of
the page, so we reset these for each plot.
(Due to the way that Sweave works each plot is a separate
environment, so the par() parameters do not endure from plot
to plot.)
The simplest labeled plot is called by using \Co{plot} and \Co{text}
without changing any of the defaults. This is useful for a first
look, but sometimes you'll want more information about each of the
nodes.
\begin{figure}
\myfig{longintro-plots1}
\caption{\Co {plot(fit); text(fit)}}
\label{plotfig1}
\end{figure}
<>=
fit <- rpart(pgstat ~ age + eet + g2 + grade + gleason + ploidy,
stagec, control = rpart.control(cp = 0.025))
par(mar = rep(0.2, 4))
plot(fit)
text(fit)
@
\begin{figure}
\myfig{longintro-plots2}
\caption{\Co {plot(fit, uniform = TRUE); text(fit,use.n = TRUE,all = TRUE)}}
\label{plotfig2}
\end{figure}
The next plot has uniform stem lengths (\Co{uniform = TRUE}), has extra
information (\Co{use.n = TRUE}) specifying number of subjects at each node,
and has labels on all the nodes, not just the terminal nodes (\Co{all = TRUE}).
<>=
par(mar = rep(0.2, 4))
plot(fit, uniform = TRUE)
text(fit, use.n = TRUE, all = TRUE)
@
\begin{figure}
\myfig{longintro-plots3}
\caption{\Co{plot(fit, branch=0); text(fit,use.n = TRUE)}}
\label{plotfig3}
\end{figure}
Fancier plots can be created by modifying the \Co{branch} option,
which controls the shape of branches that connect a node to its
children. The default for the plots is to have square shouldered trees
(\Co{branch = 1.0}). This can be taken to the other extreme with
no shoulders at all (\Co{branch=0}).
<>=
par(mar = rep(0.2, 4))
plot(fit, branch = 0)
text(fit, use.n = TRUE)
@
\begin{figure}
\myfig{longintro-plots4}
\caption{\Co{plot(fit, branch = 0.4, uniform = TRUE, compress = TRUE)}}
\label{plotfig4}
\end{figure}
These options can be combined with others to create the plot that fits your
particular needs.
The default plot may be inefficient in it's use of space: the terminal
nodes will always lie at x-coordinates of 1,2,\ldots.
The \Co{compress} option attempts to improve this by overlapping some nodes.
It has little effect on figure \ref{plotfig4}, but in figure \ref{figdig}
it allows the lowest branch to ``tuck under'' those above.
If you want to play around with the
spacing with compress, try using \Co{nspace} which regulates the space
between a terminal node and a split.
<>=
par(mar = rep(0.2, 4))
plot(fit, branch = 0.4,uniform = TRUE, compress = TRUE)
text(fit, all = TRUE, use.n = TRUE)
@
\begin{figure}
\myfig{longintro-plots5}
\caption{Fancier plot}
\label{plotfig5}
\end{figure}
Several of these options were combined into a function called
\Co{post.rpart}, whose default action of creating a .ps file
in the current directory is now rather dated.
Identical results to the function can be obtained by the collection
of options shown below, the result is displayed in figure \ref{plotfig5}.
The code is essentially
<>=
par(mar = rep(0.2, 4))
plot(fit, uniform = TRUE, branch = 0.2, compress = TRUE, margin = 0.1)
text(fit, all = TRUE, use.n = TRUE, fancy = TRUE, cex= 0.9)
@
The \Co{fancy} option of \Co{text} creates the ellipses and
rectangles, and moves the splitting rule to the midpoints of
the branches.
\Co{Margin} shrinks the plotting region
slightly so that the \Co{text} boxes don't run over the edge of the
plot.
The \Co{branch} option makes the lines exit the ellipse
at a ``good'' angle.
A separate package \texttt{rpart.plot} carries these ideas much further.
\section{Other functions}
A more general approach to cross-validation can be gained using the
\Co{xpred.rpart} function.
Given an rpart fit, a vector of $k$ complexity parameters,
and the desired number of cross-validation groups,
this function returns an $n$ by $k$ matrix containing the
predicted value $\hat y_{(-i)}$ for each subject, from the model
that was fit without that subject.
The $cp$ vector defaults to the geometric mean of the $cp$
sequence for the pruned tree on the full data set.
Here is an example that uses the mean absolute deviation for
continuous data rather than the usual mean square error.
<<>>=
carfit <- rpart(Price/1000 ~ ., cars)
carfit$cptable
price2 <- cars$Price[!is.na(cars$Price)]/1000
temp <- xpred.rpart(carfit)
errmat <- price2 - temp
abserr <- colMeans(abs(errmat))
rbind(abserr, relative=abserr/mean(abs(price2-mean(price2))))
@
We see that on the absolute error scale the relative error
improvement is not quite so large,
though this could be expected given that the optimal split was
not chosen to minimize absolute error.
\section{Test Cases}
\subsection{Classification}
The definitions for classification trees can get the most complex,
especially with respect to priors and loss matrices.
In this section we lay out a simple example, in great detail.
(This was done to debug the R functions.)
\begin{table}
\centering
\begin{tabular} {r|rrrrrrrr}
y & 1& 2 & 3 & 1 & 2 &3 & 1 & 2 \\
x1& 1 &2 & 3 & 4 & 5& 6 & 7 & 8 \\
x2& 1 &2 & 3 & 4 & 5& 6 & 1 & 2 \\
x3&NA &22& 38& 12&NA& 48& 14& 32 \\
\multicolumn{2}{c}{} \\ % tricky way to get a blank line with no vertical bar
y& 3 & 1 & 2 & 3 & 1 & 2 & 1 \\
x1& 9 & 10 & 11 & 12 & 13 & 14 & 15 \\
x2& 3 & 4 & 5 & 6 & 1 & 2 & 3 \\
x3& 40& NA & 30& 46 & 28 & 34 & 48 \\
\end{tabular}
\caption{Data set for the classification test case}
\label{datatab}
\end{table}
Let $n=15$, and the data be as given in table \ref{datatab}.
The loss matrix is defined as
$$
L = \begin{array}{ccc}
0 & 2 & 2 \\
2 & 0 & 6 \\
1 & 1 & 0 \\ \end{array}
$$
where rows represent the true class and columns the assigned class.
Thus the error in mistakenly calling a class 2 observation a 3 is quite
large in this data set.
The prior probabilities for the study are assumed to be
$\pi = .2, .3, .5$,
so class 1 is most prevalent in the input data ($n_i$=6, 5, and 4 observations,
respectively), but class 3 the most prevalent in the external population.
Splits are chosen using the Gini index with altered priors,
as defined in equation (4.15) of Breiman et al \cite{Breiman83}.
\begin{eqnarray*}
\tilde\pi_1 &=& \pi_1(0+2+2) / \sum \tilde \pi_i = 4/21 \\
\tilde\pi_2 &=& \pi_2(2+0+6) / \sum \tilde \pi_i = 12/21 \\
\tilde\pi_3 &=& \pi_3(1+1+0) / \sum \tilde \pi_i = 5/21\\
\end{eqnarray*}
For a given node $T$, the Gini impurity will be
$\sum_j p(j|T)[1-p(j|T)] = 1- \sum p(j|T)^2$,
where $p(j|T)$ is the expected proportion of class $j$ in the node:
$$
p(j|T) = \tilde\pi_j [n_i(T)/n_i ]/ \sum p(i|T)
$$.
Starting with the top node,
for each possible predicted class we have the following loss
$$
\begin{tabular}{cc}
predicted class & E(loss)\\
1 & .2*0 + .3*2 + .5*1 = 1.1 \\
2 & .2*2 + .3*0 + .5*1 = 0.9 \\
3 & .2*2 + .3*6 + .5*0 = 2.2 \\
\end{tabular}
$$
The best choice is class 2, with an expected loss of 0.9.
The Gini impurity for the node, using altered priors, is
$G = 1 - (16+144+25)/21^2 = 256/441 \approx .5805$.
Assume that variable $x1$ is split at 12.5, which is, as it turns out,
the optimal split point for that variable under the constraint that
at least 2 observations are in each terminal node.
Then the right node will have class counts of
(4,4,4) and the left node of (2,1,0).
For the right node (node 3 in the tree) \\
\begin{tabular}{rcl}
$P(R)$ &=&.2(4/6) + .3(4/5) + .5(4/4)= 131/150 \\
&=& \mbox{probability of the node (in the population)} \\
$p(i|R)$ &=& (.2(4/6), .3(4/5), .5(4/4))/$P(R)$ \\
$\widetilde P(R)$ &=& (4/21)(4/6) + (12/21)(4/5) + (5/21)(4/4)
= 259/315=37/45 \\
$\tilde p(i|R)$&=& [(4/21)(4/6), (12/21)(4/5), (5/21)(4/4)] (45/37) \\
$G(R)$&=& $1 - \sum \tilde p(i|R)^2 \approx .5832$
\end{tabular}
For the left node (node 2) \\
\begin{tabular}{rcl}
$P(L)$ &=&.2(2/6) + .3(1/5) + .5(0/4)= 19/150 \\
$p(i|L)$ &=& (.4/3, .3/5, 0)/ $P(L)$ = (10/19, 9/19, 0) \\
$\widetilde P(L)$ &=& $1- \widetilde P(R) =8/45$ \\
$\tilde p(i|L)$ &=& [(4/21)(2/6), (12/21)(1/5), (5/21)(0/4)]/
$\widetilde P(L)$\\
$G(L)$ &=&$ 1 - \sum \tilde p(i|L)^2 \approx .459$ \\
\end{tabular}
The total improvement for the split involves the
change in impurity between the parent and the two child nodes
$$
n(G - [\widetilde P(L)* G(L) + \widetilde P(R)* G(R)] )
\approx .2905
$$
where $n=15$ is the total sample size.
For variable $x2$ the best split occurs at 5.5, splitting the data
into left and right nodes with class counts of (2,2,1) and
(4,3,3), respectively.
Computations just exactly like the above give an improvement of
1.912.
For variable $x3$ there are 3 missing values, and the computations are
similar to what would be found for a node further down the tree with
only 12/15 observations.
The best split point is at 3.6, giving class counts of
(3,4,0) and (1,0,4) in the left and right nodes, respectively.
For the right node (node 3 in the tree)\\
\begin{tabular}{rcl}
$\widetilde P(R)$ &=& (4/21)(3/6) + (12/21)(4/5) + (5/21)(0/4) = 174/315\\
$\tilde p(i|R)$&=& [(4/21)(3/6), (12/21)(4/5), (5/21)(0/4)] (315/174) \\
&=& (5/29, 24/29, 0) \\
$G(R)$&=& $1 - (25+576)/29^2 = 240/841 approx .2854$
\end{tabular}
For the left node (node 2) \\
\begin{tabular}{rcl}
$\widetilde P(L)$ &=& (4/21)(1/6) +(12/21)(0/5) + (5/21)(4/4)= 85/315\\
$\tilde p(i|L)$ &=& [(4/21)(1/6), (12/21)(0/5), (5/21)(4/4)] (315/85) \\
&=& (2/17, 0, 15/17)\\
$G(L)$ &=&$ 1 - (4 + 225)/17^2 = 60/289 \approx .2076$ \\
\end{tabular}
The overall impurity for the node involves only 12 of the 15
observations, giving the following values for the top node:\\
\begin{tabular}{rcl}
$\widetilde P(T)$ &=& 174/315 + 85/315 = 259/315\\
$\tilde p(i|T)$ &=& [(4/21)(4/6), (12/21)(4/5), (5/21)(4/4)] (315/259) \\
&=& (40/259, 144/259, 75/259) \\
$G(T)$& =& $1- (40^2 + 144^2 + 75^2)/259^2 = 39120/67081$
\end{tabular}
The total improvement for the split involves the impurity $G$ of all three
nodes, weighted by the probabilities of the nodes under the alternate priors.
$$
15* \{(259/315)(39120/67081) - [(174/315)(240/841) + (85/315)(60/289)]\}
\approx 3.9876
$$
As is true in general with the rpart routines, variables with missing
values are penalized with respect to choosing a split -- a factor of
259/315 or about 12/15 in the case of $x3$ at the top node.
Table \ref{tab:cutpoint} shows the statistics for all of the cutpoints
of $x3$.
\begin{table}
\centering
\begin{tabular}{c | rrrrr}
Cutpoint& $P(L)$ & $P(R)$ & $G(L)$ & $G(R)$ & $\Delta I$\\ \hline
1.3 & 0.03 & 0.97 & 0.00 & 0.56 & 0.55\\
1.8 & 0.06 & 0.94 & 0.00 & 0.53 & 1.14\\
2.5 & 0.18 & 0.82 & 0.46 & 0.57 & 0.45\\
2.9 & 0.21 & 0.79 & 0.50 & 0.53 & 0.73\\
3.1 & 0.32 & 0.68 & 0.42 & 0.56 & 1.01\\
3.3 & 0.44 & 0.56 & 0.34 & 0.52 & 1.96\\
3.6 & 0.55 & 0.45 & 0.29 & 0.21 & 3.99\\
3.9 & 0.61 & 0.39 & 0.41 & 0.26 & 2.64\\
4.3 & 0.67 & 0.33 & 0.48 & 0.33 & 1.56\\
4.7 & 0.73 & 0.27 & 0.53 & 0.45 & 0.74\\
4.8 & 0.79 & 0.21 & 0.56 & 0.00 & 0.55\\
\end{tabular}
\caption{Cut points and statistics for variable $x3$, top node}
\label{tab:cutpoint}
\end{table}
Because $x3$ has missing values, the next step is choice of a surrogate
split. Priors and losses currently play no role in the computations for
selecting surrogates. For all the prior computations, the effect of
priors is identical to that of adding case weights to the observations
such that the apparent frequencies are equal to the chosen priors;
since surrogate computations do account for case weights, one could argue
that they should also then make use of priors.
The argument has not yet been found compelling enough to add this to the
code.
Note to me: the cp is not correct for usesurrogate=0. The error
after a split is not (left error + right error) -- it also needs to
have a term (parent error for those obs that weren't split).
\bibliographystyle{plain}
\bibliography{refer}
\end{document}