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Re: st: Statistical/Stata question


From   Constantine Daskalakis <C_Daskalakis@mail.jci.tju.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Statistical/Stata question
Date   Tue, 17 Feb 2004 21:07:42 -0500

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 (letīs 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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