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st: specifying offset in logistic regression


From   Kathryn L Adeney <kathryn7@u.washington.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: specifying offset in logistic regression
Date   Sat, 11 Jul 2009 10:55:58 -0700 (PDT)

Hello,

I am trying to account for time-at-risk ("risktime") as an offset variable in a logistic regression model.  As far as I understand, it should not matter whether I use the option "exposure(risktime)" or "offset(lnrisktime)", where lnrisktime = ln(risktime), in the glm.  However, I get two different results depending on how I specify the option.  Is this a glitch that anyone else has come across?  This is the basic format of the 2 codes:

glm y x1 x2 x3 , fam(binom) link(logit) offset(lnrisktime)

OR

glm y x1 x2 x3 , fam(binom) link(logit) exposure(risktime)

Thanks for your help,
-Kathryn

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