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st: offset variables


From   frone@ria.buffalo.edu
To   statalist@hsphsun2.harvard.edu
Subject   st: offset variables
Date   Mon, 1 Jul 2002 19:38:33 -0400

I'm reading a matched case-control study that used conditional logistic 
regression (in STATA) to explore the predictors of being injured at work 
during the prior 12 months.  The analysis uses number of days worked 
during the period as an offset variable presumably to adjust for 
differential exposure.  In conditional logistic regression the coefficient 
for the offset is constrained to equal 1.

I don't understand why one would make this constraint, as opposed to using 
number of days worked as another predictor variable.  Is there a reference 
that explains the various uses for offset variables?  I wasn't able to 
find a useful explanation in the STATA archives or through a general 
search of the net.

Thanks in advance for any help.

Mike Frone
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