dampack: gamma_params – R documentation

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gamma_params

Calculate shape and scale (or rate) parameters of a gamma distribution.

Description

Function to calculate the shape, α, and scale, θ, (or rate, β) parameters of a gamma distribution based on the method of moments (MoM) using the mean μ and standard deviation σ of the random variable of interest.

Usage

gamma_params(mu, sigma, scale = TRUE)

Arguments

`mu`	scalar with the mean of the random variable.
`sigma`	scalar with the standard deviation of the random variable.
`scale`	logical variable indicating scale parameterization of the gamma distribution (Default is TRUE). If FALSE, rate parameterization is retrieved

Value

A list contianing the following:

shape Shape parameter of gamma distribution

scale Scale parameter of gamma distribution (If scale=TRUE)

rate Rate parameter of gamma distribution (If scale=FALSE)

Details

Based on method of moments. If μ is the mean and σ is the standard deviation of the random variable, then the the shape, α, scale, θ, and rate, β, parameters are computed as follows

α=\frac{μ^2}{σ^2},

θ = \frac{σ^2}{μ}

and

β = \frac{μ}{σ^2}

References

Gamma distribution. (2018, February 7). In Wikipedia, The Free Encyclopedia. Retrieved 17:23, February 11, 2018, from https://en.wikipedia.org/w/index.php?title=Gamma_distribution&oldid=824541785

Examples

mu    <- 2
sigma <- 1
# Scale specification
gamma_params(mu, sigma)
# Rate specification
gamma_params(mu, sigma, scale = FALSE)

dampack

Decision-Analytic Modeling Package

v1.0.0

GPL-3

Authors

Fernando Alarid-Escudero [aut], Greg Knowlton [aut, cre], Caleb Easterly [aut], Eva Enns [aut]

Initial release