[R-sig-ME] Parametrization of the beta-binomial model in glmmadmb {glmmADMB}

[Torbjørn Ergon]

Dear list,

I want to do some simulation based goodness-of-fit assessment of a 
(zero-inflated) beta-binomial model fitted with glmmadmb, but I realize 
I don't quite understand how this model is parameterized.

I understand that the linear predictor is 
inverse-logit(alfa/(alpha+beta)) where alpha and beta are the parameters 
of the beta-distribution (right?). But what is the exact definition of 
"Beta-binomial dispersion parameter" (fit.glmmadmb$alpha) expressed in 
terms of the parameters of the beta distribution (alpha and beta)?

Cheers,

Torbjørn Ergon
University of Oslo