Rdistance (version 4.3.0)

GammaReparam: 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)

Value

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

Arguments

shp

Rdistance's shape parameter

scl

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

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")