I was under the impression that the tree method (e.g. as implemented in rpart) was insensitive to monotonic transformations of the dependent variable. e.g. Breiman Olshen et al. Classification and Regression Trees state "In a standard data structure [a tree] is invariant under all monotone transformations of individual ordered varaibles" (p. 57)
However, I get very different results from tr.hh.pri <- rpart((log(YPRISX+1)~AGE+DRUGUSEY+SEX+OBSXNUM)) and tr.hh.pri <- rpart(YPRISX~AGE+DRUGUSEY+SEX+OBSXNUM) the former gives more splits and different splits. Some notes: The DV is a count variable, and highly skew, with some 0s, many 1s, and a long right tail out to 99. AGE ranges from 18-25 DRUGUSEY is ordered (hardest drug used) and OBSXNUM is also ordered (proportion of your friends who object to your having 'casual sex') printing the first tree gives 1) root 307 23.472040 0.7114605 2) AGE>=19.5 196 13.811070 0.6857971 4) OBSXNUM< 2.5 69 5.712526 0.6338252 8) DRUGUSEY>=1.5 15 2.261203 0.5161601 * 9) DRUGUSEY< 1.5 54 3.185960 0.6665100 * 5) OBSXNUM>=2.5 127 7.810911 0.7140339 * 3) AGE< 19.5 111 9.303947 0.7567761 6) DRUGUSEY< 0.5 48 1.105266 0.6727132 * 7) DRUGUSEY>=0.5 63 7.601052 0.8208239 14) SEX>=1.5 21 1.258395 0.7317629 * 15) SEX< 1.5 42 6.092803 0.8653544 * printing the second tree gives 1) root 307 144.540700 1.1205210 2) AGE>=19.5 196 68.382650 1.0561220 * 3) AGE< 19.5 111 73.909910 1.2342340 6) DRUGUSEY< 0.5 48 2.979167 0.9791667 * 7) DRUGUSEY>=0.5 63 65.428570 1.4285710 14) SEX>=1.5 21 6.571429 1.1428570 * 15) SEX< 1.5 42 56.285710 1.5714290 * So, is this the 'exception that proves the rule'? Have I done something wrong? Or what? Any ideas or thoughts? Thanks in advance Peter Peter L. Flom, PhD Assistant Director, Statistics and Data Analysis Core Center for Drug Use and HIV Research National Development and Research Institutes 71 W. 23rd St www.peterflom.com New York, NY 10010 (212) 845-4485 (voice) (917) 438-0894 (fax) ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help