rpart puzzle
Two problems here: 1) rpart is supposed to follow the Breiman et al (1984) monograph, which looks at all n*v values of potential splitters (n = cases; v= variables) and then splits on the midpoint using the rule: x7<= 37 x7 > 37 2) It makes the tree useless for dealing with unknown observations where x7 may happen to equal 37. The reason this even came to my notice is because of this precise circumstance. I found that rpart moved to a surrogate variable when x7=37. There was no need to use a surrogate since x7 wasn't missing and assumed a value well within x7's range.
At 11:22 AM 7/12/2001 -0700, White.Denis at epamail.epa.gov wrote:
I haven't looked that carefully at rpart, but in tree the potential splits
are midpoints between actual data values. So if x7 had values of 36 and
38, but not 37, a valid split would be < 37 and > 37.
Marc
Feldesman
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Sent
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puzzle
th.ethz.ch
07/12/2001
09:02
I've been using the package rpart with R 1.3.0 for Windows to produce
simple classification trees for some measurement data from paleontological
specimens. Both the rpart documentation and the output confirm that the
program produces splits on continuous data that leave "holes" in the
data. It is probably of little practical importance, but is there a reason
why the binary splits are constructed in the form (e.g):
x7 < 37
x7 > 37
as opposed to the actual CART (tm) methodology of:
x7 <= 37
x7 > 37
It seems to me that if one were to use rpart to classify an unknown case
where x7 = 37, the program wouldn't actually know which way to move the
case.
I've read through the rpart technical report, the rpart user's manual, the
rpart help file and see this practice illustrated, but don't find any
explanation for this minor (and probably trivial) departure from the
methodology illustrated in the CART program and in the Breiman et al book.
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