[R] logistic regression and dummy variable coding

[Seyed Reza Jafarzadeh]

NewVar <- relevel( factor(OldVar), ref = "b")
should create a dummy variable, and change the reference category for the model.

Reza


On 6/28/07, Bingshan Li <bli1 at bcm.tmc.edu> wrote:
> 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!
>
>
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>