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Re: st: question on zero inflated regression


From   Nicola Man <n.man@unsw.edu.au>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: question on zero inflated regression
Date   Thu, 17 Feb 2011 18:26:18 +1100

I have a different take on the decision to drop the zero-inflated model if there are no variables predicting excess zeros.  For example, in parasite egg count data which I have been working on, it is known that generally a zero-inflated model may fit better.  I might have no predictors for the excess zeros, but the zero-inflation model still works better because the fit to the neg bin distribution is better than that of the standard model.  I would use the vuong test to test whether the zero-inflated model is better than the standard model for this decision on which distributional model to take.

Regards,
Nicola

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Date: Sun, 13 Feb 2011 13:41:13 -0800
From: Owen Gallupe <ogallupe@gmail.com>
Subject: Re: st: question on zero inflated regression

Hi,

Seems to me that there are a number of ways you can go with this:

1) if you don't have a theoretical justification as to why there is an
inflation of zeros, put in all variables as potential causes.
2) don't use the zero inflated model at all. If there is reason to
suspect that the zero scores are simply part of the continuum of
scores (as in, the same causal factors related to scores of 1, 2,
etc., are the same factors as scores of zero but in varying degrees),
then a standard negative binomial might be better.

In my opinion, this is a theoretical question.

Owen Gallupe


On Sun, Feb 13, 2011 at 10:58 AM, rachel grant
<rachelannegrant@gmail.com> wrote:
> Hi I am new to the list and pretty clueless.
> I am trying to use Zero inflated models, my data are counts of
> ampibians arriving at a breeding site per night. Most nights none
> arrive or one,  but some nights many arrive hence the data are
> overdispersed and have excess zeroes. I am using zero inflated
> negative binomial regression. What I am confused about is I have to
> specify the inflation variable (ie the one that is generating the
> excess zeroes). I have 7 predictor variable and have no clue which is
> responsible for generating the zeroes, so what to put into the model
> as the inflation variable? Thanks for reading this
>
> --
> regards, Rachel
>
> Rachel Grant
> Dept. Life Sciences
> Open University
> UK
>
>
>
>
> --
> regards, Rachel
>
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