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

Simone Vincenzi svincenz at nemo.unipr.it
Fri Dec 2 10:29:53 CET 2005

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
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)


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 at nemo.unipr.it 


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