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st: offset term in logit


From   Leslie Witter <leslie_witter@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: offset term in logit
Date   Sun, 26 Sep 2010 10:01:50 -0700 (PDT)

Hi,
I ran a logit in stata with an offset term for the natural log of duration of exposure...offset(lnduration).  I'm trying to recreate stata's predictions doing the calculations by hand.  I understand that by including the offset term, the offset variable is constrained to one.  Where do you plug that in to the logistic equation?  I'm calculating the predicted probability using the following equations, but my predictions don't match those generated by stata:

Z1=a+bx1+cx2+.... 
then, P(1)=exp(Z1)/(1+exp(Z1))

Thanks for your help!
Leslie

---------------------------------------
Leslie Witter
M.Sc. candidate - NRES Biology
University of Northern British Columbia
3333 University Way
Prince George, BC  V2N 4Z9
(250)960-6739
email:witter@unbc.ca
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