[R] rpart puzzle

[Marc Feldesman]

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.






=====================
Dr. Marc R. Feldesman
Professor and Chairman
Anthropology Department
Portland State University
1721 SW Broadway
Portland, Oregon 97201
email:  feldesmanm at pdx.edu
phone:  503-725-3081
fax:    503-725-3905
http://web.pdx.edu/~h1mf
PGP Key Available On Request
======================

"Anyway, no drug, not even alcohol, causes the fundamental ills of society.
If we're looking for the source of our troubles, we shouldn't test people
for drugs, we should test them for stupidity, ignorance, greed and love of
power."   P.J. O'Rourke

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