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How do I go about selecting the ideal location to use for pruning the tree here?

Or maybe someone can explain to me in simple language what this output means. I see that rel_error is constantly decreasing when I run this but x_error decreases until the 3rd point at .48 before rising again.

Which point do I use to pick the best length for the tree?

 library(MASS)
 library(part)
 z=rpart(crim~., data = Boston, method="anova", xval=10, cp=.0005)
 printcp(z)
 plotcp(z)
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http://cran.r-project.org/web/packages/rpart/rpart.pdf plotcp() details: "The set of possible cost-complexity prunings of a tree from a nested set. For the geometric means of the intervals of values of cp for which a pruning is optimal, a cross-validation has (usually) been done in the initial construction by rpart. The cptable in the fit contains the mean and standard deviation of the errors in the cross-validated prediction against each of the geometric means, and these are plotted by this function. A good choice of cp for pruning is often the leftmost value for which the mean lies below the horizontal line." PS library(rpart)

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