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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 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

* * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: poisson exposure problem***From:*"Austin Nichols" <austinnichols@gmail.com>

**References**:**st: poisson exposure problem***From:*"Philip Ender" <ender97@gmail.com>

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