[R] GLMs: Negative Binomial family in R?

[nflynn@ualberta.ca]

Greetings R Users!

I have a data set of count responses for which I have made repeated observations
on the experimental units (stream reaches) over two air photo dates, hence the
mixed effect.  I have been using Dr. Jim Lindsey's GLMM function found in his
"repeated" measures package with the "poisson" family.

My problem though is that I don't think the poisson distribution is the right
one to discribe my data which is overdispersed; the variance is greater than
the mean.  I have read that the "negative binomial" regression models can
account for some of the differences among observations by adding in a error
term that independent of the the covariates.

I haven't yet come across a mixed effects model that can use the "negative
binomial" distribution.

If any of you know of such a function - I will certainly look forward to hearing
from you!  Additionally, if any of you have insight on zero-inflated data, and
testing for this, I'd be interested in your comments too.  I'll post a summary
of your responses to this list.

Best Regards,
Nadele Flynn, M.Sc. candidate.
University of Alberta