dbetabinom versions

February 17, 2013

(This article was first published on Xi'an's Og » R, and kindly contributed to R-bloggers)

I got this email from a student:

(1) I used the following R function in package “emdbook“
dbetabinom(x,prob,size,theta, shape1,shape2,log=FALSE)
more precisely I did
curve(dbetabinom(x,size=15,shape1=3,shape2=7),ylim=c(0,.12),xlim=c(0,10))
(2) instead I use the following R function in package “VGAM“
dbetabinom.ab(x,size,shape1,shape2,log=FALSE,.dontuse.prob=NULL)
more precisely I did
curve(dbetabinom.ab(x,size=15,shape1=3,shape2=7),ylim=c(0,.12),xlim=c(0,10))
and I get two different curves! Sad!

to which I replied only the following

> dbetabinom.ab(1:10,size=15,shape1=3,shape2=7)
[1] 0.08893281 0.12450593 0.14198045 0.14198045 0.12861758
[6] 0.10718132 0.08268273 0.05905909 0.03886795 0.02332077
> dbetabinom(1:10,size=15,shape1=3,shape2=7)
[1] 0.08893281 0.12450593 0.14198045 0.14198045 0.12861758
[6] 0.10718132 0.08268273 0.05905909 0.03886795 0.02332077

as the beta-binomial density is only defined for integers! (emdbook is the R package associated with Benjamin Bolker’s Ecological Models and Data in R.)

Filed under: R, Statistics, University life Tagged: bet-binomial distribution, emdbook, R, VGAM