R/betaABfromMeanSD.R

In ndbrown6/lazypuppy: Doing Bayesian Data Analysis 2nd Edition

'betaABfromMeanSD' <- function(mean,
							   sd)
{
	if (mean<=0 | mean>=1) {
		stop("must have 0 < mean < 1")
	}
	if (sd<=0) {
		stop("sd must be > 0")
	}
	kappa = mean*(1-mean)/sd^2 - 1
	if (kappa<=0) {
		stop("invalid combination of mean and sd")
	}
	a = mean*kappa
	b = (1.0-mean)*kappa
	return(invisible(list(a=a, b=b)))
}