Hi Frank,
 
I do not quite get you. What do you mean by simulation and speed issues? I do not see why they have to be considered in logistic regression.
 
Thanks.
 
Bingshan

________________________________

From: Frank E Harrell Jr [mailto:f.harrell@vanderbilt.edu]
Sent: Fri 6/29/2007 7:40 AM
To: Li, Bingshan
Cc: Seyed Reza Jafarzadeh; r-help@stat.math.ethz.ch
Subject: Re: [R] logistic regression and dummy variable coding



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@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!
>>>
>>>
>>>
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>>>
>>> ______________________________________________
>>> R-help@stat.math.ethz.ch mailing list
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>>> guide.html
>>> and provide commented, minimal, self-contained, reproducible code.
>>>
>
> ______________________________________________
> R-help@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.
>


--
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University



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