[R] logistic regression and dummy variable coding
Frank E Harrell Jr
f.harrell at vanderbilt.edu
Fri Jun 29 14:40:58 CEST 2007
Bingshan Li wrote:
> Hi All,
> Now it works. Thanks for all your answers and the explanations are
> very clear.
But note that you are not using R correctly unless you are doing a
simulation and have some special speed issues. Let the model functions
do all this for you.
> On Jun 28, 2007, at 7:44 PM, Seyed Reza Jafarzadeh wrote:
>> NewVar <- relevel( factor(OldVar), ref = "b")
>> should create a dummy variable, and change the reference category
>> for the model.
>> 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,
>>> 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?
>>> [[alternative HTML version deleted]]
>>> R-help at stat.math.ethz.ch mailing list
>>> PLEASE do read the posting guide http://www.R-project.org/posting-
>>> and provide commented, minimal, self-contained, reproducible code.
> R-help at stat.math.ethz.ch mailing list
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University
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