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Re: st: question on zero inflated regression


From   rachel grant <rachelannegrant@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: question on zero inflated regression
Date   Mon, 14 Feb 2011 10:04:02 +0000

Thanks for responding Owen, that is really helpful. I tried running
the regression using multiple predictors as inflation variables and
two were highly significant. (minimum temperature and degreedays:
degreedays being a derivative of minimum temperature). This is
biologically a sensible result because amphibians cannot move if the
temperature is low, hence generating a zero result. My question now
is: does the significant p value indicate that the zeroes are
generated by low minimum temperatures? If so this is a very
interesting observation. Is it now legitimate to run the model using
only min temp and degree days as inflation variables. Many thanks -
things are becomimg clearer. Rachel

Rachel Grant
Dept. Life Sciences
Open University
UK

On 13 February 2011 21:41, Owen Gallupe <ogallupe@gmail.com> wrote:
> 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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-- 
regards, Rachel

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