R help - Jun 2005 - function for "two-part" or "two-condition" models

Hello,

This is an (hopefully) improved question of one I posted several weeks
ago. Does anyone know of a function for fitting "two-part" models?
These models are designed to handle count data with so many zeroes
that they can't be fit well with zero-inflated Poisson models or other
'typical' GLMs. My understanding is that they work by first fitting a
binomial model to separate the zeros from the occurrences (positive
integers) before fitting a Poisson model to account for variation in
abundance. 

I have tried help.search("two-part") and many other similar guesses.

Thanks,
Richard

-- 
Richard Chandler, M.S. Candidate
Department of Natural Resources Conservation
UMass Amherst
(413)545-1237

Hi Richard,

I'm not sure that I can imagine how data can have too many zeros to be
fit well with zero-inflated Poisson models. Won't the excess zeros be
accommodated by increasing the the inflation?

In any case, if you want a model that separates the zeros from the
occurrences before fitting a Poisson model to account for variation in
abundance then it might be safest to do that split manually.

Another angle to try is to treat it as a special case of a finite
mixture regression.  I think that some of Jim Lindsey's code will fit
such models. Google can help you find his wbsite.

An MS student of mine explored these models for regeneration modeling.
I'd be happy to send you a pdf of his thesis if it would help.

Cheers,

Andrew

On Mon, Jun 27, 2005 at 03:35:30PM -0400, Richard Chandler
wrote:
> Hello,
> 
> This is an (hopefully) improved question of one I posted several weeks
> ago. Does anyone know of a function for fitting "two-part"
models?
> These models are designed to handle count data with so many zeroes
> that they can't be fit well with zero-inflated Poisson models or other
> 'typical' GLMs. My understanding is that they work by first fitting
a
> binomial model to separate the zeros from the occurrences (positive
> integers) before fitting a Poisson model to account for variation in
> abundance. 
> 
> I have tried help.search("two-part") and many other similar
guesses.
> 
> Thanks,
> Richard
> 
> -- 
> Richard Chandler, M.S. Candidate
> Department of Natural Resources Conservation
> UMass Amherst
> (413)545-1237
> 
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-- 
Andrew Robinson                      Ph: 208 885 7115
Department of Forest Resources       Fa: 208 885 6226
University of Idaho                  E : andrewr at uidaho.edu
PO Box 441133                        W : http://www.uidaho.edu/~andrewr
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