[R] Export chaid decision tree rules

Achim Zeileis Achim.Zeileis at uibk.ac.at
Sat Jan 3 21:58:26 CET 2015

On Sat, 3 Jan 2015, David Winsemius wrote:

> On Jan 3, 2015, at 1:21 AM, Rodica Coderie via R-help wrote:
>> Hello,
>> Can the decisions tree rules be exported? Along with the probabilities associated with each node?For example, I've created a CHAID decision with a target variable RESPONSE (YES/NO). I have 17 inner nodes with 19 terminal nodes. How which terminal node has the highest probability of YES and which is the probability?
>> An example of a terminal node output is below:clicks_flag in YES: NO (n = 1142, err = 5.3%)
>> Thanks!Rodica
>> 	[[alternative HTML version deleted]]
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Yes, a reproducible example would have been good.

I assume you are talking about the "CHAID" package from R-Forge. We 
haven't got a nice and ready to use function in "partykit" (which "CHAID" 
is built upon) but we have an unexported .list.rules.party function 
which does a good part of what you want to do.

## package and data
ucb <- as.data.frame(UCBAdmissions)
ucb <- ucb[rep(1:nrow(ucb), ucb$Freq), 1:3]

## fit tree
ch <- chaid(Admit ~ Gender + Dept, data = ucb)

## get rule path

And with that information it is not too hard to set something up that is 
close to what you want, I think:

format_rules <- function(object, ...) {
   ft <- fitted(object)
   ns <- tapply(ft[[2]], ft[[1]], length)
   pr <- tapply(ft[[2]], ft[[1]], function(y)
   lb <- tapply(ft[[2]], ft[[1]], function(y)
     names(sort(table(y), decreasing = TRUE))[1])
   rl <- partykit:::.list.rules.party(object)
   paste0(rl, ": ", lb, " (n = ", ns, ", ", round(100 * pr, 2), "%)")


> -- 
> David Winsemius
> Alameda, CA, USA
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