R help - Nov 2010 - Offset in glm poisson using R vs Exposure in Stata

R-helpers,

I am hoping to find someone who uses both R and program Stata for GLMs.

I am a beginner R user, finding my own way through; learning code etc. at the
same time as learning the statistics I need to complete my project.

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:   
glm(Count~md+ms+rf+sg+offset(Eff),family=poisson,data=DepthHabGen)

(Making use of glm{stats})

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.

So now I defer to those with more experience, does anyone know how to produce
the equivalent of Stata's glm  'exposure' in R?

Thank you in advance,
Columbine
 		 	   		  
	[[alternative HTML version deleted]]

Columbine Caroline Waring <caquilegia <at> hotmail.com> writes:
> I am hoping to find someone who uses both R and program Stata for GLMs.
[snip]
> 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:  
 
glm(Count~md+ms+rf+sg+offset(Eff),family=poisson,data=DepthHabGen)
> 
> (Making use of glm{stats})
> 
  Based on your discussion above did you mean

glm(count~md+ms+rf+sg+offset(log(Eff)),
   family=poisson,data=DepthHabGen)

 ? 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.