overdispersion in GLMMs (Alejandro Mart?nez Abra?n)
Elizabeth and Alejandro-- We discuss the over-dispersed Poisson mixed model (with per-observation random effect) in the following tutorial: Atkins, D. C., Baldwin, S., Zheng, C., Gallop, R. J., & Neighbors, C. (in press). A tutorial on count regression and zero-altered count models for longitudinal substance use data. Psychology of Addictive Behaviors. which you can find along with data and R code: http://depts.washington.edu/cshrb/newweb/statstutorials.html Hope that helps. cheers, Dave > 1. How can I estimate overdispersion in a Poisson GLMMM? > I usually fit overdispersed Poisson (or Binomial) GLMMs by adding a unique identifier for each observation, then adding that unique ID as a random term. You can "test" how overdispersed the model is by looking at the standard deviation assoicated with that random effect, or by comparing the fit of a model with the overdispersion term to one without it. Alternatively, you can look at overdispersion due to random effects of individual, plot, etc, using the same basic procedure. I think I got the idea from the Gelman et al. Bayesian stats text. I would be curious to know if others do this also. > > 2. I am trying to run a quasipoisson GLMM using the lmer function and the > lme4 library but I get a > warning stating that "glmer cannot deal with quasi error families? Any tip? > because I have seen this done. > My understanding is that this functionality has been removed, since it is +/- redundant with the approach used above, but less naturally linked to the mixed model framework. -- **************************** Elizabeth E. Crone Senior Ecologist, Harvard Forest Harvard University Petersham MA 01366 office: (978)756-6145 main: (978)724-3302 cell: (406)531-3498 FAX: (978)724-3595 email: ecrone at fas.harvard.edu
Dave Atkins, PhD Research Associate Professor Department of Psychiatry and Behavioral Science University of Washington datkins at u.washington.edu Center for the Study of Health and Risk Behaviors (CSHRB) 1100 NE 45th Street, Suite 300 Seattle, WA 98105 206-616-3879 http://depts.washington.edu/cshrb/ (Mon-Wed) Center for Healthcare Improvement, for Addictions, Mental Illness, Medically Vulnerable Populations (CHAMMP) 325 9th Avenue, 2HH-15 Box 359911 Seattle, WA 98104 http://www.chammp.org (Thurs)