xpred.rpart.RdGives the predicted values for an rpart fit, under
cross validation, for a set of complexity parameter values.
xpred.rpart(fit, xval = 10, cp, return.all = FALSE)
| fit | a object of class |
|---|---|
| xval | number of cross-validation groups. This may also be an explicit list of integers that define the cross-validation groups. |
| cp | the desired list of complexity values. By default it is taken from the
|
| return.all | if FALSE return only the first element of the prediction |
A matrix with one row for each observation and one column for each complexity
value. If return.all is TRUE and the prediction for each node
is a vector, then the result will be an array containing all of the
predictions. When the response is categorical, for instance, the
result contains the predicted class followed by the class
probabilities of the selected terminal node;
result[1,,] will be the matrix of predicted classes,
result[2,,] the matrix of class 1 probabilities, etc.
Complexity penalties are actually ranges, not values. If the
cp values found in the table were \(.36\), \(.28\),
and \(.13\), for instance, this means that the first row of the
table holds for all complexity penalties in the range \([.36, 1]\),
the second row for cp in the range \([.28, .36)\) and
the third row for \([.13,.28)\). By default, the geometric mean
of each interval is used for cross validation.
fit <- rpart(Mileage ~ Weight, car.test.frame) xmat <- xpred.rpart(fit) xerr <- (xmat - car.test.frame$Mileage)^2 apply(xerr, 2, sum) # cross-validated error estimate#> 0.79767456 0.28300396 0.04154257 0.01132626 #> 1426.3025 776.0628 616.4675 602.6129# approx same result as rel. error from printcp(fit) apply(xerr, 2, sum)/var(car.test.frame$Mileage)#> 0.79767456 0.28300396 0.04154257 0.01132626 #> 62.12379 33.80206 26.85075 26.24730printcp(fit)#> #> Regression tree: #> rpart(formula = Mileage ~ Weight, data = car.test.frame) #> #> Variables actually used in tree construction: #> [1] Weight #> #> Root node error: 1354.6/60 = 22.576 #> #> n= 60 #> #> CP nsplit rel error xerror xstd #> 1 0.595349 0 1.00000 1.04186 0.175789 #> 2 0.134528 1 0.40465 0.55562 0.103815 #> 3 0.012828 2 0.27012 0.41228 0.077063 #> 4 0.010000 3 0.25729 0.41012 0.070133