[R] One inflated Poisson or Negative Binomal regression

[Peter Flom]

Hello

I am interested in Poisson or (ideally) Negative Binomial regression
with an inflated number of 1  responses

I have seen JK Lindsey's fmr function in the gnlm library, which fits
zero inflated Poisson (ZIP) or zero inflated negative binomial
regression, but the help file states that for ' Poisson or related
distributions  the mixture involves the zero category'.

I had thought of perhaps subtracting 1 from all the counts and then
fitting the ZIP or ZINB models, and then adding 1, but am not sure if
this is legitimate, or if there is some better method.

Contextual details:
The dependent variable is number of primary sexual partners in the last
year.  The independent variables include a) Being married or in a
committed relationship  b) using hard drugs  c) sex  d) age

N is c. 500

Not surprisingly, there are a large number of 1 responses, especially
for those who are married or in a relationship.  More surprisingly, the
mean number of partners is the same (1.05 vs. 1.02) for people in and
not in relationships, but the variances are very different, mostly
because those in a relationhsip are much more likely to say exactly 1.

Thanks in advance

Peter

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
www.peterflom.com
New York, NY 10010
(212) 845-4485 (voice)
(917) 438-0894 (fax)