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: bbol...@gmail.com > To: caquile...@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]] ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
