[R] logistic regression and dummy variable coding

From: Bingshan Li <bli1_at_bcm.tmc.edu>
Date: Thu, 28 Jun 2007 18:16:38 -0500


Hello everyone,

I have a variable with several categories and I want to convert this into dummy variables and do logistic regression on it. I used model.matrix to create dummy variables but it always picked the smallest one as the reference. For example,

model.matrix(~.,data=as.data.frame(letters[1:5]))

will code 'a' as '0 0 0 0'. But I want to code another category as reference, say 'b'. How to do it in R using model.matrix? Is there other way to do it if model.matrix has no such functionality?

Thanks!

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