negbinomGetSizeProb {simstudy}

Convert negative binomial mean and dispersion parameters to size and prob parameters

Description

Convert negative binomial mean and dispersion parameters to size and prob parameters

Usage

negbinomGetSizeProb(mean, dispersion)

Arguments

mean

The mean of a gamma distribution

dispersion

The dispersion parameter of a gamma distribution

Details

In simstudy, users specify the negative binomial distribution as a function of two parameters - a mean and dispersion. In this case, the variance of the specified distribution is mean + (mean^2)*dispersion. The base R function rnbinom uses the size and prob parameters to specify the negative binomial distribution. This function converts the mean and dispersion into the size and probability parameters.

Value

A list that includes the size and prob parameters of the neg binom distribution

Examples

set.seed(12345)
mean <- 5
dispersion <- 0.5
sp <- negbinomGetSizeProb(mean, dispersion)
c(sp$size, sp$prob)
vec <- rnbinom(1000, size = sp$size, prob = sp$prob)
(estMoments <- c(mean(vec), var(vec)))
(theoryMoments <- c(mean, mean + mean^2 * dispersion))
(theoryMoments <- c(sp$size * (1 - sp$prob) / sp$prob, sp$size * (1 - sp$prob) / sp$prob^2))