[R] regression tree xerror

[Sherri Miller]

I am running some models (for the first time) using rpart and am getting
results I don't know how to interpret. I'm using cross-validation to prune
the tree and the results look like:
Root node error: 172.71/292 = 0.59148

n= 292

         CP nsplit rel error  xerror     xstd
1  0.124662      0   1.00000 1.00731 0.093701
2  0.064634      1   0.87534 1.08076 0.092337
3  0.057300      2   0.81070 1.07684 0.095582
4  0.038462      4   0.69610 0.99104 0.091659
5  0.036200      5   0.65764 1.01596 0.094635
6  0.029228      7   0.58524 1.00058 0.095440
7  0.028779      8   0.55601 1.00704 0.093242
8  0.024192      9   0.52724 0.97844 0.088936
9  0.018038     11   0.47885 1.02749 0.092263
10 0.016867     13   0.44278 1.08704 0.092112
11 0.015465     14   0.42591 1.10805 0.097813
12 0.015000     15   0.41044 1.11130 0.097881

I do not understand why the rel error rate is going down, but the xerror
generally goes up. For some of the runs, the xerror never goes down. Is
result caused by something in my data structure? I have run some example
datasets from the various help manuals and the xerror goes down, as one
would expect. Any suggestions?

Sherri

Sherri L. Miller
Wildlife Biologist
Redwood Sciences Laboratory
707.825.2949
707.825.2901 (FAX)