GammaReparam: Reparameterise Gamma parameters for use in dgamma

In Rdistance: Density and Abundance from Distance-Sampling Surveys

Reparameterise Gamma parameters for use in dgamma

Description

Transform Rdistance's version of the Gamma distribution parameters, which is that of Becker and Quan, into the version for use in R::dgamma() and elsewhere.

Usage

GammaReparam(shp, scl)

Arguments

shp	Rdistance's shape parameter
scl	Rdistance's scale parameter. This parameter is related to covariates via exp(x'B).

Value

A list with components $shp and $scl, which are the re-parameterized versions of the input parameters suitable for us in R::dgamma().

See Also

'Details' section of Gamma.like for Rdistance's Gamma distribution

Examples

# Rdistance Gamma parameters
Rd.scl <- 50  # must be >0
Rd.shp <- 1.5 # must be >1

# dgamma parameters
dgParams <- GammaReparam(Rd.shp, Rd.scl)
dgParams

# Gamma distribution with (Rd.scl, Rd.shp) from 0 to 100
curve(dgamma(x, shape=dgParams$shp, scale = dgParams$scl)
           , from = 0
           , to = 100)
 
# Rdistance's version: same curve but scaled so maximum = 1
x <- seq(0, 100, length = 200) 
scl <- dgParams$scl
shp <- dgParams$shp
m <- (shp - 1) * scl
g <- (x / m)^(shp - 1) * exp(-(x - m) / scl) # distance function
plot(x, g, type = "l")

Rdistance documentation built on Jan. 10, 2026, 1:07 a.m.