20.8 Interpret RPart Decision Tree

{#dtrees:sec:explain_read_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 No and 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 1). observations and a default decision of No. There are 42 observations with Yes as the decision, so these are `lost'' if we make the decisionNofor all observations. The probability ofNois reported as $0.83593750$ (which is $214/256$) and ofYes` is $0.16406250$ ($42/256$).

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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