simstudy (version 0.7.0)

gammaGetShapeRate: Convert gamma mean and dispersion parameters to shape and rate parameters

Description

Convert gamma mean and dispersion parameters to shape and rate parameters

Usage

gammaGetShapeRate(mean, dispersion)

Value

A list that includes the shape and rate parameters of the gamma distribution

Arguments

mean

The mean of a gamma distribution

dispersion

The dispersion parameter of a gamma distribution

Details

In simstudy, users specify the gamma distribution as a function of two parameters - a mean and dispersion. In this case, the variance of the specified distribution is (mean^2)*dispersion. The base R function rgamma uses the shape and rate parameters to specify the gamma distribution. This function converts the mean and dispersion into the shape and rate.

Examples

set.seed(12345)
mean <- 5
dispersion <- 1.5
rs <- gammaGetShapeRate(mean, dispersion)
c(rs$shape, rs$rate)
vec <- rgamma(1000, shape = rs$shape, rate = rs$rate)
(estMoments <- c(mean(vec), var(vec)))
(theoryMoments <- c(mean, mean^2 * dispersion))
(theoryMoments <- c(rs$shape / rs$rate, rs$shape / rs$rate^2))