Poisson mixed models
Hi Anna, So you tried a GLMM with quasipoisson and a GLM with Poisson? How about a GLMM with Poisson? Sounds like you may have a random effect that is necessary for your hypothesis test, but which does not explain any variation (but I really have no way of knowing). Hank
On Oct 21, 2008, at 5:33 AM, Renwick, A. R. wrote:
Dear All There has been a lot of talk recently on this forum regarding (over) dispersion and quasi models. I am running a GLMM with a poisson family for some tick burden data I have and I wanted to check if I had overdispersion in my model (and thus a poisson family would be inappropriate). The only method I have found to do this is to run the model with a quasipoisson family and then ask for the scale parameter using: lme4:::sigma(model) However, when I do this my model appears severely UNDER dispersed: sigmaML 3.779694e-06 Without the random effect in the model (i.e a GLM) the scale parameter is 1.07 - almost perfect for a poisson family. Is the method I am trying not appropriate to determine the dispersion in the mixed model? Does anyone know a better method? Many thanks, Anna The University of Aberdeen is a charity registered in Scotland, No SC013683.
_______________________________________________ R-sig-mixed-models at r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-sig-mixed-models
Dr. Hank Stevens, Associate Professor 338 Pearson Hall Botany Department Miami University Oxford, OH 45056 Office: (513) 529-4206 Lab: (513) 529-4262 FAX: (513) 529-4243 http://www.cas.muohio.edu/~stevenmh/ http://www.users.muohio.edu/harkesae/ http://www.cas.muohio.edu/ecology http://www.muohio.edu/botany/ "E Pluribus Unum" "I love deadlines. I love the whooshing noise they make as they go by." (Douglas Adams) If you send an attachment, please try to send it in a format anyone can read, such as PDF, text, Open Document Format, HTML, or RTF. Why? See: http://www.gnu.org/philosophy/no-word-attachments.html