Re: st: Statistical/Stata question

[Constantine Daskalakis <C_Daskalakis@mail.jci.tju.edu>]

At 08:26 PM 2/17/2004, Jose Maria wrote:
> Dear Statalisters:
> I think it is more of a statistical question, but may I ask it:
> If I have a categorical variable  (let�s say with 4 categories) and run 
> the logistic regression _logistic depvar category2 category3 category4_, I 
> get the odds ratios relating each of the 2,3,4 categories  against the 
> category1 reference, with straigthway interpretation. What is the 
> interpretation if I do _logistic depvar category3 category4_?
> Sorry if it is a silly question.
> Regards,
> Jose Maria
It isn't silly at all.

In effect, you are forcing the first two categories (1 and 2) to have 
exactly the same odds (probability) for the event. In other words, you are 
constraining the odds ratio for category2 vs. category1 to be exactly equal 
to 1. As a consequence, the comparison of category3 to category1 has the 
same odds ratio as the comparison of category3 to category2 (and similarly 
for category4). For the purposes of the model, there are only 3 categories: 
a combined 1/2 category, category3 and category4.

CD




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________________________________________________________________

Constantine Daskalakis, ScD
Assistant Professor,
Biostatistics Section, Thomas Jefferson University,
211 S. 9th St. #602, Philadelphia, PA 19107
   Tel: 215-955-5695
   Fax: 215-503-3804
   Email: c_daskalakis@mail.jci.tju.edu
   Webpage: http://www.jefferson.edu/medicine/pharmacology/bio/  

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