[R-sig-ME] glmmadmb Negative binomial dispersion parameter

Marte Lilleeng mlilleeng at gmail.com
Tue Feb 23 15:02:48 CET 2016

Hello everyone.
I wonder if you can help me with the following questions?

1) How is the Negative binomial dispersion parameter calculated in glmmadmb?

2) Is there,as for mixed effects poisson models (glmer), a *rule of thumb*
for when the dispersion parameter is representing trouble (overdispersion)?
I learned from Zuur and Ileno that for a mixed effects poisson mod the
dispersion is ok as long as it is not above 1.3-1.4(calculated this way;

E <- residuals (modelname)

pb <- length(fixef(modelname)+1) # +1 due to random intercept variance

overdisp <- sum(E^2)/(nrow(dataset)-pb)

3) How do you recommend to do the model validation for glmmadmb with
negative binomial error structure?

Best regards from
"New to mixed models with NB", Marte Lilleeng

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