questions on rpart (tree changes when rearrange the order of covariates?!)

Greetings,

I am using rpart for classification with "class" method. The test data  is
the Indian diabetes data from package mlbench.

I fitted a classification tree firstly using the original data, and then
exchanged the order of Body mass and Plasma glucose which are the
strongest/important variables in the growing phase. The second tree is a
little different from the first one. The misclassification tables are
different too. I did not change the data, but why the results are so
different?

Does anyone know how rpart deal with ties?

Here is the codes for running the two trees.


library(mlbench)
data(PimaIndiansDiabetes2)
mydata<-PimaIndiansDiabetes2
library(rpart)
fit2<-rpart(diabetes~., data=mydata,method="class")
plot(fit2,uniform=T,main="CART for original data")
text(fit2,use.n=T,cex=0.6)
printcp(fit2)
table(predict(fit2,type="class"),mydata$diabetes)
## misclassifcation table: rows are fitted class
      neg pos
  neg 437  68
  pos  63 200
#Klimt(fit2,mydata)

pmydata<-data.frame(mydata[,c(1,6,3,4,5,2,7,8,9)])
fit3<-rpart(diabetes~., data=pmydata,method="class")
plot(fit3,uniform=T,main="CART after exchaging mass & glucose")
text(fit3,use.n=T,cex=0.6)
printcp(fit3)
table(predict(fit3,type="class"),pmydata$diabetes)
##after exchage the order of BODY mass and PLASMA glucose
      neg pos
  neg 436  64
  pos  64 204
#Klimt(fit3,pmydata)


Thanks,


--------------------------------------------------------------------------------------
Yuanyuan Huang

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