Hi again,
Apologies for the garbled symbols in my previous post. Here is a correct version:

Hi all,
I＊m using glmmADMB to fit a mixed model with a beta famiily, and I would like some clarification on how to interpret the parameter estimates. Specifically, I would like to use the results as input into the R function ＆dbeta＊, but I＊m not sure what in the glmmadmb output corresponds to the shape1 and shape2 parameters of dbeta. I＊m assuming the ＆alpha＊, ＆b＊ and ＆stdbeta＊ components are the ones to go for. But which is which?
According to the dbeta help page, shape1 and shape2 corresponds to alpha and beta respectively in the beta density function:

gamma(alpha+beta)/(gamma(alpha)gamma(beta))x^(alpha-1)(1-x)^(beta-1)

But how do these correspond to the ＆alpha＊ (dispersion parameter) and ＆b＊ in the glmmadmb output? I get the most realistic  results (when comparing to the distribution of my data) if I let the the ＆alpha＊ corresponds to shape2 (i.e. 乒 in the above expression) and ＆b＊ to shape1 (丟 in the expression). Maybe I＊m only being confused by the nomenclature? But if I＊ve understood the maths behind the beta distribution properly, when shape1 is greater than shape2  the distribution is left-skewed, and the dispersion is determined by the ratio shape2/shape1. And vice versa for a right-skewed distribution. So how can I determine which goes in as shape1 and shape2 when calculating my beta densities?

Thanks for any help!

Martin


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Martin Biuw (PhD)
Researcher

Norwegian Institute for Nature Research - NINA
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