[R-sig-ME] Dispersion parameter negbin1 in glmmADMB

[Dean Castillo]

HI Hanna,

It may be very well that GLM and glmm ADMB calculate the
overdispersion parameter in a different way, but the quasipoisson and
negative binomial are not the same.
Quasipoisson uses a mean regression function (like normal poisson) but
leaves the overdispersion parameter unrestricted. Negative binomial
can be represented as a gamma mixture of poisson distributions.

The pscl package has some documentation explaining different types of
count models. It is called: "Regression models for count data in R".
It is useful to help understand count models.

Dean

On Tue, Mar 27, 2012 at 8:58 AM, Hanna ten Brink
<hannatenbrink at gmail.com> wrote:
> Dear R users
>
> I am trying to understand how the dispersion parameter in the glmmADBM
> package is calculated for the negbin1 family.
> Is it correct that the negbin1 family is the same as the quasipoisson
> family?
> Because when I run a simple model in GLM(family=quasipoisson) or in
> glmmADMB(family=negbin) for the Owls-dataset, it gives different dispersion
> parameters.
>
> e.g.
>
> ADBM_binom1 <- glmmadmb(NCalls~(FoodTreatment+ArrivalTime)*SexParent+
> offset(logBroodSize),
> data=Owls,
> zeroInflation=FALSE,
> family="nbinom1")
>
> Dispersion parameter=8.2014
>
> GLM_quasipois <- glm(NCalls~(FoodTreatment+ArrivalTime)*SexParent+
> offset(logBroodSize),
> data=Owls,
> family=quasipoisson)
>
> Dispersion parameter= 6.259856
>
> Does this mean that the negbin1 and quasipoisson family are not the same?
> Or does the glmmADMB package calculates the dispersion parameter in a
> different way?
>
> Thank you!
>
> Hanna ten Brink
>
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