negbinomGetSizeProb | R Documentation |
Convert negative binomial mean and dispersion parameters to size and prob parameters
negbinomGetSizeProb(mean, dispersion)
mean |
The mean of a gamma distribution |
dispersion |
The dispersion parameter of a gamma distribution |
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.
A list that includes the size and prob parameters of the neg binom distribution
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))
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