[R] RE: zero-inflated count models (was polr problem solved)

Peter Flom flom at ndri.org
Sat Oct 9 18:00:22 CEST 2004

John Fox wrote
>From your description, it seems possible that there are too many zeros
for a Poisson or negative-binomial model. Since the focus of your paper
is the methodology, you might want to try a zero-inflated Poisson or
negative-binomial model. Though I haven't tried them, I'm aware of two
sources of R functions for zero-inflated count models -- zeroinfl(),
from Simon Jackman's web site <http://pscl.stanford.edu/content.html>,
and gnlr()
in Jim Lindsey's gnlm package, which is not available on CRAN but at

Indeed. I am going to use these, as well, and attempt to demonstrate
that fitting the correct model not only is correct, statistically, but
also give results that are substantively different than the 'usual'
models.  I hope that this will increase the use of appropriate models in
the field (which, in case anyone is interested, is substance abuse
research, and, in my particular case, the syringe-sharing behaviors of
drug injectors).


Peter L. Flom, PhD
Assistant Director, Statistics and Data Analysis Core
Center for Drug Use and HIV Research
National Development and Research Institutes
71 W. 23rd St
New York, NY 10010
(212) 845-4485 (voice)
(917) 438-0894 (fax)

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