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st: RE: Negative binomial regression and Incident rate ratio(IRR)


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: Negative binomial regression and Incident rate ratio(IRR)
Date   Tue, 1 Sep 2009 13:35:34 +0100

Counts but no denominators means no ratios can be calculated, to my simple mind. That is, modelling without an offset implies that whatever you are comparing is exactly comparable. The results are unreliable, to put it gently, to the extent that that isn't true. How could it be otherwise? 

(N.B. inciden_ce_ rate ratios.)

Nick 
n.j.cox@durham.ac.uk 

Muhammad Riaz

I am trying to fit a model for count data using Negative binomial regression, it gives incident rate ratios for the variables in the model, one of the options for this type of model is specifying the offset variable while fitting the model in stata using glm. This offset should be the person years (if i am correct). however I do not have data on this variable. My question is:
 
Is it correct to fit the model without an offset and would the Incident rate ratio be still correct to report If I do not specify the offset variable in the model?
 

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