Rev Language Reference

dnGamma(RealPos shape, RealPos rate)

shape :	RealPos (pass by const reference)
	The shape parameter.
rate :	RealPos (pass by const reference)
	The rate parameter (rate = 1/scale).

RealPos

The gamma distribution takes two parameters, shape and rate. Similar to how 1/rate gives the mean of the exponential, shape/rate gives the mean of the gamma. It provides a natural prior distribution for parameters that could be considered as sums of exponential variables. The probability density function is f(x) = rate^shape * x^(shape - 1) * e^(-rate * x) / Gamma(shape), where Gamma is the gamma function. Note that for shape = 1, the gamma distribution reduces to an exponential distribution.

# lets simulate
a <- rgamma(1000,shape=4,rate=4)
# we expect a mean of 1
mean(a)

# create a random variable
x ~ dnGamma(shape=4,rate=1)
x

• dnExponential