Dear Mark, I think you want glm(DV ~ log10(IV), family=poisson) Note that the poisson family uses the log-link by default. Hence you don't need to log-transform DV yourself.
Best regards, Thierry ---------------------------------------------------------------------------- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek team Biometrie & Kwaliteitszorg Gaverstraat 4 9500 Geraardsbergen Belgium Research Institute for Nature and Forest team Biometrics & Quality Assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 thierry.onkel...@inbo.be www.inbo.be To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey > -----Oorspronkelijk bericht----- > Van: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > Namens Mark Na > Verzonden: vrijdag 1 juli 2011 23:10 > Aan: r-help@r-project.org > Onderwerp: [R] Poisson GLM with a logged dependent variable...just asking for > trouble? > > Dear R-helpers, > > I'm using a GLM with poisson errors to model integer count data as a function > of > one non-integer covariate. > > The model formula is: log(DV) ~ glm(log(IV,10),family=poisson). > > I'm getting a warning because the logged DV is no longer an integer. > > I have three questions: > > 1) Can I ignore the warning, or is logging the DV (resulting in > non-integers) a serious violation of the Poisson error structure? > > 2) If the answer to #1 is "no, don't ignore it, it's serious" then can I use a > quasipoisson error structure instead (does not give the same > warning) and if so are there any pitfalls to using the quasipoisson model? Are > there any better alternatives for count data where the counts must be logged? > Or, should I just abandon logging the DV? In that case, how could I compare > the > fit of a Poisson model (without logging the DV) to that of a GLM with normal > errors (with a logged DV). AIC would not be valid because the DVs are > different, > right? > > 3) The quasipoisson model doesn't return an AIC value. Why, and is there > anything I can do to calculate AIC manually, that would allow me to compare > this model to other models? > > Many thanks in advance for your help! > > Cheers, Mark > > ______________________________________________ > 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. ______________________________________________ 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.
