Re: st: poisson exposure problem

[Steven Samuels <sjhsamuels@earthlink.net>]

Phil,

Some thoughts:

I'd want to know what the 'observations' are: different times, areas?

Although posited as a Poisson problem, this is a problem in  
predicting proportions between 0 & 1, since the student  is willing  
to condition on 'exposure' equal to the number of seeds of all  
plants. I would suggest a  random effects binomial regression model  
like -xtmelogit- or -glogit-.  In either case, the cases with no  
seeds cannot be used.


I'd recommend a preliminary analysis to predict the total number of  
seeds with one of Stata's count procedures, including -xtmepoisson- ,  
-xtnbreg- . This analysis could separate out  out influences on total  
numbers of seeds from influences on the proportion belonging to the  
species of interest.   This preliminary analysis could predict the  
zero counts of seeds.

A more advanced model- could predict the relative and absolute  
numbers of more than two species, distinguishing between separate and  
common influences.  To me another question is: why expect a Poisson  
distribution at all? If seeds are generated 'locally', then there  
will be an unmeasured source of variation within areas, namely the  
number of plants of each species.

-Steve


On Mar 27, 2008, at 4:19 PM, Philip Ender wrote:


> A student comes in with a poisson model.  The response variable is the
> number of seeds of a certain species.  There is an exposure variable
> which is the total seeds of all species.  The problem is that there
> are six exposure values of zero.  There are three other predictor
> variables and 72 total observations.  Is there any way of dealing with
> this problem other than dropping those six values?  Any suggestions?
>
> --
> Phil Ender
> Statistical Consulting Group
> UCLA Academic Technology Services
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