21.8 Interpret RPart Decision Tree
## n= 256 ## ## node), split, n, loss, yval, (yprob) ## * denotes terminal node ## ## 1) root 256 42 No (0.83593750 0.16406250) ## 2) Cloud3pm< 6.5 180 11 No (0.93888889 0.06111111) * ## 3) Cloud3pm>=6.5 76 31 No (0.59210526 0.40789474) ## 6) Pressure3pm>=1016 34 5 No (0.85294118 0.14705882) * ## 7) Pressure3pm< 1016 42 16 Yes (0.38095238 0.61904762) ## 14) WindDir3pm=ESE,SSE,W,WNW,WSW 17 5 No (0.70588235 0.29411765) * ## 15) WindDir3pm=ENE,N,NE,NNE,NNW,NW,S 25 4 Yes (0.16000000 0.84000000) *
The textual version of a classification decision tree is reported by rpart.
The legend, which begins with
node) indicates that each node
is identified by a number, followed by a split (which will usually be
in the form of a test on the value of a variable), the number of
observations \(n\) at that node, the number of observations that are
incorrectly classified (the \(loss\)), the default classification for
the node (the \(yval\)), and then the distribution of classes in that
node (the \(yprobs\)) across
Yes. The next
line indicates that a ``*’’ denotes a terminal node of the tree (i.e.,
a leaf node—the tree is not split any further at that node).
The actual tree starts with the root node labelled
observations and a default decision of
No. There are 42 observations with
Yes as the
decision, so these are `
lost'' if we make the decisionNo
for all observations. The probability ofNo
is reported as $0.83593750$ (which is $214/256$) and ofYes` is \(0.16406250\)
The root node is split into two branches, nodes number 2 and 3. For
node number 2, the split corresponds to those observations for which
Cloud3pm is less than \(6.5\). This accounts for 180 observations
and whilst 11 of them are
Yes the majority (with a proportion
of \(0.93888889\)) are
No. We can read the remainder of the
tree similarly. Node 3 is split into two other nodes, the second of
which is split further until the terminal nodes.
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