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Re: st: poisson exposure problem


From   Steven Samuels <sjhsamuels@earthlink.net>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: poisson exposure problem
Date   Fri, 28 Mar 2008 10:16:39 -0400

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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