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


From   Ronan Conroy <rconroy@rcsi.ie>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Negative binomial regression and Incident rate ratio(IRR)
Date   Wed, 2 Sep 2009 11:13:30 +0100

On 1 MFómh 2009, at 11:22, Muhammad Riaz wrote:

Dear All,
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?

There are two scenarios: the first is that you know the exposure time. This might happen because, for example, everyone in the study had been observed for the same length of time. In this case, you can just create a variable with the information. Or you might assume that all participants had been observed for the same period and give it an arbitrary value (1, say) - you would have to defend this assumption, of course.

The second scenario is that you don't know the length of time for which each person was observed, and it cannot be assumed to be the same. In this case, you cannot calculate incidence (which relies on a measure of person*time as its denominator).



I will really appreciate any help.

Thank you,

M Riaz





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Ronan Conroy
=================================

rconroy@rcsi.ie
Royal College of Surgeons in Ireland
Epidemiology Department,
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