betaGetShapes: Convert beta mean and precision parameters to two shape...

In simstudy: Simulation of Study Data

Convert beta mean and precision parameters to two shape parameters

Description

Convert beta mean and precision parameters to two shape parameters

Usage

betaGetShapes(mean, precision)

Arguments

mean	The mean of a beta distribution
precision	The precision parameter (phi) of a beta distribution

Details

In simstudy, users specify the beta distribution as a function of two parameters - a mean and precision, where 0 < mean < 1 and precision > 0. In this case, the variance of the specified distribution is (mean)*(1-mean)/(1+precision). The base R function rbeta uses the two shape parameters to specify the beta distribution. This function converts the mean and precision into the shape1 and shape2 parameters.

Value

A list that includes the shape parameters of the beta distribution

Examples

set.seed(12345)
mean <- 0.3
precision <- 1.6
rs <- betaGetShapes(mean, precision)
c(rs$shape1, rs$shape2)
vec <- rbeta(1000, shape1 = rs$shape1, shape2 = rs$shape2)
(estMoments <- c(mean(vec), var(vec)))
(theoryMoments <- c(mean, mean * (1 - mean) / (1 + precision)))
(theoryMoments <- with(rs, c(
  shape1 / (shape1 + shape2),
  (shape1 * shape2) / ((shape1 + shape2)^2 * (1 + shape1 + shape2))
)))

simstudy documentation built on Nov. 23, 2023, 1:06 a.m.