
Ben,

Thank you for your assistance.
Going back to basics and using the data set as you suggested has resulted in a win.

Set A works! 
using   +offset(log(variable))   or ,offest=(log(Eff))   is the same as using exposure(variable) program stata.

I went back and isloated a problem with code before this point.

Thank you again.

Columbine


> Date: Tue, 16 Nov 2010 17:47:23 -0500
> From: bbolker@gmail.com
> To: caquilegia@hotmail.com
> CC: r-help@r-project.org
> Subject: Re: [R] Offset in glm poisson using R vs Exposure in Stata
> 
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 11/16/2010 03:08 PM, Columbine Caroline Waring wrote:
> 
> > Officially I tried:
> 
> **A**
> >> glm(count~md+ms+rf+sg+offset(log(Eff)), family=poisson,data=DepthHabGen)
> >> glm(count~md+ms+rf+sg, offset=(log(Eff)), family=poisson,data=DepthHabGen)
> > (which of course are the same as eachother)
> > 
> 
> **B**
> >> glm(count~md+ms+rf+sg, offset=(Eff), family=poisson,data=DepthHabGen)
> >> glm(count~md+ms+rf+sg+offset(Eff), family=poisson,data=DepthHabGen)
> > (which are also the same between themselves, yet wrong compared to the
> > STATA model)
> > 
> > Additionally, given the text you found on stata website, which I am
> > familiar with, I also tried:
> 
> **C**
> >> glm(count~md+ms+rf+sg, offset=(exp(Eff)), family=poisson,data=DepthHabGen)
> >> glm(count~md+ms+rf+sg+offset(exp(Eff)), family=poisson,data=DepthHabGen)
> > (which still might be the solution however R issues the following response:
> > Error: no valid set of coefficients has been found: please supply
> > starting value)
> 
>   In my opinion, B and C are just wrong (C is in the wrong direction,
> and it's not surprising that glm has hiccups when adding a
> doubly-exponentiated version of the Eff variable to the linear predictor).
> 
>   So I think all the other stuff about specifying starting values is
> essentially a red herring.
> 
>   I still don't know what Stata is doing but in your position I would
> make up some data where I knew the answer and try it in both R and
> Stata.  For example:
> 
> 
> set.seed(1001)
> md <- runif(100)
> ms <- runif(100)
> dat <- expand.grid(md=md,ms=ms)
> dat$eff <- runif(nrow(dat))+2*dat$md
> dat$eta <- with(dat,2*md-2*ms+log(eff))
> dat$y <- with(dat,rpois(nrow(dat),exp(eta)))
> 
> m1 <- glm(y~md+ms+offset(log(eff)),data=dat, family="poisson")
> summary(m1)
> 
>   I have purposely set up the offset here so that it is strongly
> correlated with md, and will screw things up if it is not accounted for
> properly.  I made the data set quite large so that it is clear that the
> model is accurately retrieving the coefficients (2 and -2) assigned to
> the predictors.
> 
>   cheers
>     Ben
> 
> -----BEGIN PGP SIGNATURE-----
> Version: GnuPG v1.4.10 (GNU/Linux)
> Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/
> 
> iEYEARECAAYFAkzjCfsACgkQc5UpGjwzenOc+wCfTMK8AdbiFkraQeDTd1LMcqOf
> 1dAAmgP/bR72ELMHsmAYHcPM2IX0AWLN
> =bnkm
> -----END PGP SIGNATURE-----
 		 	   		  
	[[alternative HTML version deleted]]

