[R] logistic regression and dummy variable coding

[Frank E Harrell Jr]

Bingshan Li wrote:
> Hi All,
> 
> Now it works. Thanks for all your answers and the explanations are  
> very clear.
> 
> Bingshan

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.

Frank

> 
> 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.
>>
>> 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]]
>>>
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> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
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> 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