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

In simstudy: Simulation of Study Data

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)

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.

Value

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

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))

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