[R] Offset in glm poisson using R vs Exposure in Stata

Ben Bolker bbolker at gmail.com
Tue Nov 16 14:16:20 CET 2010

Columbine Caroline Waring <caquilegia <at> hotmail.com> writes:

> I am hoping to find someone who uses both R and program Stata for GLMs.
> What I have is the code from Stata  and am trying to reproduce the same
analysis in R - my program of choice.
> .         glm count md ms rf sg, family(poisson)
> exposure(effort) eform
> I am lost at the point of finding the equivalent code for 'exposure'. 
> Having looked at a few forums and 'googled'. I thought 'offset', used as   
offset=(log(Eff))   or the
> equivalent    +offset(log(Eff))   would produce the desired effect.
> Incidentally my code was:  
> (Making use of glm{stats})

  Based on your discussion above did you mean


 ? is this just a typo ?

according to http://www.stata.com/help.cgi?glm

 exposure(varname)         include ln(varname) in model with coefficient
                                  constrained to 1
 offset(varname)           include varname in model with coefficient
                                  constrained to 1

> However, offset does not seem to be equivalent to 'exposure' in Stata. As
coefficients and log likelhood
> estimates differ.
> So I asked the following questions:
> 1. Do both programs produce the same results without
> 'exposure' i.e. glm models 
> Yes,  log likelihoods and coefficients are the same.
> 2. How about using the unintuitive non logged " offset=Eff" ?
> Coefficients and log likelihoods still differ.

  You're not doing anything obviously wrong.  Are you sure your
"effort" and "Eff" variables are the same, i.e. nothing got mangled
moving to R?

  I don't use Stata, perhaps someone else can try.  Posting a small
reproducible example would be helpful.

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