[R] Zero-inflated neg.bin. model and pscl package

From: Simone Vincenzi <svincenz_at_nemo.unipr.it>
Date: Fri 02 Dec 2005 - 20:29:53 EST

Dear list,
I'm currently trying to develop a model to assess clam yield potential in a lagoon. I'm using the zeroinfl function of the pscl package to fit a Zero-inflated negative binomial model, given the high occurrence of zero counts.
I don't understand from the sentence in the pscl guide "Zero-inflated count models are a type of two-component mixture model, with a component for zero counts, and the other component for the positive counts" if: a)to get true estimate of the relative mean abundance, the model multiply the relative mean abundance at a site by the probability that the relative mean abundance at a site is generated through a negative binomial distribution, as proposed by Lambert (Technometrics, 1992). By using this kind of mixture model, zeros arise from one or two processes and their related covariates.
b) we have two independent models, where the first part is a binary outcome model and the second one is a negative binomial model, assuming that zeros arise from a single process and set of covariates, as proposed by Dobbie and Welsh (Austr.N.Z.J.Stat., 2001)

Thanks

Simone Vincenzi
PhD student in Ecology, University of Parma, Italy      



Simone Vincenzi, PhD Student
Department of Environmental Sciences
University of Parma
Parco Area delle Scienze, 33/A, 43100 Parma, Italy Phone: +39 0521 905696
Fax: +39 0521 906611
e.mail: svincenz@nemo.unipr.it
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Received on Fri Dec 02 20:55:33 2005

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