Hi I'm a postgrad in ecology, and have recently started to use R. I'm planning to model various sets of animal abundance (i.e. count) data in relation to habitat data using glm's and/or glmmPQL's. However, some of my potential response variables have many zeros. From what I gather the "family = ..." option in the command line does not allow for the direct specification of a truncated / zero-modified poisson distribution. My problem is probably not uncommon, but I could not find anything on truncated poisson distributions in the R web pages or documentation. Does anyone know of a bit of software that someone may have written to deal with this problem? I'm interested in modifications to the poisson as well as quasipoisson families. Thanks a lot for your help - Joern.
truncated poisson in glm / glmmPQL
2 messages · Joern Fischer, Brian Ripley
The short answer is the the model(s) you want to fit are not glms, so neither glm() nor glmmPQL() can be adapted (easily) to fit them. I think your first task is understand what a glm is. Then I suggest specifying precisely what you do want and using maximum-likelihood estimation (e.g. via optim).
On Tue, 8 Apr 2003, Joern Fischer wrote:
I'm a postgrad in ecology, and have recently started to use R. I'm planning to model various sets of animal abundance (i.e. count) data in relation to habitat data using glm's and/or glmmPQL's. However, some of my potential response variables have many zeros. From what I gather the "family = ..." option in the command line does not allow for the direct specification of a truncated / zero-modified poisson distribution. My problem is probably not uncommon, but I could not find anything on truncated poisson distributions in the R web pages or documentation. Does anyone know of a bit of software that someone may have written to deal with this problem? I'm interested in modifications to the poisson as well as quasipoisson families.
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595