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 ¥á og ¥â respectively in:
¥Ã(¥á+¥â)/(¥Ã(¥á)¥Ã(¥â))x^( ¥á -1)(1-x)^( ¥â-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. ¥â in the above expression) and ¡®b¡¯ to shape1 (¥á 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
Postal address: Framsenteret, 9296 Troms©ª, NORWAY
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