From: Ben Bolker <bolker_at_zoo.ufl.edu>

Date: Sun 04 Dec 2005 - 13:43:12 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Dec 04 13:54:49 2005

Date: Sun 04 Dec 2005 - 13:43:12 EST

Simone Vincenzi <svincenz <at> nemo.unipr.it> writes:

> 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
**>
*

From your descriptions and a quick look at the two papers you cite,
my main conclusion is that I don't really think these are actually
different models -- just different descriptions of the same statistical
model.

Digging into the code in the package and looking at
?zeroinfl shows that the package is fitting a binomial
model for the probability of a structural zero and
a Poisson or negative binomial for the result otherwise ...
you can specify covariates for both models.

As a minor point: both references you cite actually focus on ZI Poisson (not NB) regression models, although Dobbie and Welsh do allow for overdispersion ...

hope that helps

Ben Bolker

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sun Dec 04 13:54:49 2005

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