R/gamma_parameters.R

In ThomasWilli/politantheme: Provides a theme for the politan.ch blog

##' Calculate shape and rate for gamma distribution from mean, mode and standard deviation.
##'
##' 
##'
##' @export



gamma_mean <- function(mean , sd) {
  if ( mean <=0 ) stop("mean must be > 0")
  if ( sd <=0 ) stop("sd must be > 0")
  shape = mean^2/sd^2
  rate = mean/sd^2
  return( list( shape=shape , rate=rate ) )
}


gamma_mode <- function( mode , sd ) {
  if ( mode <=0 ) stop("mode must be > 0")
  if ( sd <=0 ) stop("sd must be > 0")
  rate = ( mode + sqrt( mode^2 + 4 * sd^2 ) ) / ( 2 * sd^2 )
  shape = 1 + mode * rate
  return( list( shape=shape , rate=rate ) )
}