I ran this code (several times) from the Quick-R web page (
http://www.statmethods.net/advstats/cart.html) but my cross-validation
errors increase instead of decrease (same thing happens with an unrelated
data set).

Why does this happen?
Am I doing something wrong?

# Classification Tree with rpart
library(rpart)

# grow tree
fit <- rpart(Kyphosis ~ Age + Number + Start,
   method="class", data=kyphosis)

printcp(fit) # display the results
plotcp(fit) # visualize cross-validation results


Thank you,
Claudia

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