[R] One inflated Poisson or Negative Binomal regression

[Paul Johnson]

Dear Peter:

I notice there is a R code for a Zero-inflated Poisson/NB process on the 
Stanford Political Science Computational Lab (Prof. Simon Jackman) web 
page.  If I were wanting to do  a one-inflated model, I would start with 
that because, at least to my eye, it is very easy to  follow.  Mind you, 
I did not try this myself, but I bet you could make it go.  In the file 
zeroinfl.r, look at the function:

 zeroinflNegBin <- function(parms){

it is pretty clear you'd have to supply a probability model for the 
outcomes valued 1 and then fit them into the overall likelihood.

pj


http://pscl.stanford.edu/content.html
Peter Flom wrote:

>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
>  
>

-- 
Paul E. Johnson                       email: pauljohn at ku.edu
Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
1541 Lilac Lane, Rm 504                              
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66044-3177           FAX: (785) 864-5700