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Re: st: poisson exposure problem


From   "Austin Nichols" <austinnichols@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: poisson exposure problem
Date   Fri, 28 Mar 2008 10:34:49 -0400

Philip Ender and Steven Samuels:

I think this is fine to model using a Poisson regression, though a
fractional logit (Papke and Wooldridge 1996) and other models are also
possibilities.  But I think the cases with zero exposure supply no
information, in the context of the model specified, and are rightly
dropped.  These are like cases with no observations, and can therefore
supply no information to form estimates about the rate at which events
happen.

Leslie E. Papke and Jeffrey M. Wooldridge. 1996. "Econometric Methods
for Fractional Response Variables with an Application to 401(k) Plan
Participation Rates." Journal of Applied Econometrics, 11(6): 619-632.
[see also http://www.nber.org/papers/t0147.pdf]

On Fri, Mar 28, 2008 at 10:16 AM, Steven Samuels
<sjhsamuels@earthlink.net> wrote:

>  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.

>  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?
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