gamma2 {VGAM}R Documentation

2-parameter Gamma Distribution

Description

Estimates the 2-parameter gamma distribution by maximum likelihood estimation.

Usage

gamma2(lmu = "loge", lshape = "loge",
       emu = list(), eshape = list(),
       method.init = 1, deviance.arg = FALSE,
       ishape = NULL, zero = -2)

Arguments

lmu, lshape Link functions applied to the (positive) mu and shape parameters (called mu and shape respectively). See Links for more choices.
emu, eshape List. Extra argument for each of the links. See earg in Links for general information.
ishape Optional initial value for shape. A NULL means a value is computed internally. If a failure to converge occurs, try using this argument. This argument is ignored if used within cqo; see the iShape argument of qrrvglm.control instead.
method.init An integer with value 1 or 2 which specifies the initialization method for the mu parameter. If failure to converge occurs try another value (and/or specify a value for ishape).
deviance.arg Logical. If TRUE, the deviance function is attached to the object. Under ordinary circumstances, it should be left alone because it really assumes the shape parameter is at the maximum likelihood estimate. Consequently, one cannot use that criterion to minimize within the IRLS algorithm. It should be set TRUE only when used with cqo under the fast algorithm.
zero Integer valued vector, usually assigned -2 or 2 if used at all. Specifies which of the two linear/additive predictors are modelled as an intercept only. By default, the shape parameter (after lshape is applied) is modelled as a single unknown number that is estimated. It can be modelled as a function of the explanatory variables by setting zero=NULL. A negative value means that the value is recycled, so setting -2 means all shape parameters are intercept only.

Details

This distribution can model continuous skewed responses. The density function is given by

f(y;mu,shape) = exp(-shape * y / mu) y^(shape-1) shape^(shape) / [mu^(shape) * gamma(shape)]

for mu > 0, shape > 0 and y > 0. Here, gamma() is the gamma function, as in gamma. The mean of Y is mu=mu (returned as the fitted values) with variance sigma^2 = mu^2 / shape. If 0<shape<1 then the density has a pole at the origin and decreases monotonically as y increases. If shape=1 then this corresponds to the exponential distribution. If shape>1 then the density is zero at the origin and is unimodal with mode at y = mu - mu / shape; this can be achieved with lshape="loglog".

By default, the two linear/additive predictors are eta1=log(mu) and eta2=log(shape). This family function implements Fisher scoring and the working weight matrices are diagonal.

This VGAM family function handles multivariate responses, so that a matrix can be used as the response. The number of columns is the number of species, say, and zero=-2 means that all species have a shape parameter equalling a (different) intercept only.

Value

An object of class "vglmff" (see vglmff-class). The object is used by modelling functions such as vglm and vgam.

Note

The response must be strictly positive. A moment estimator for the shape parameter may be implemented in the future.

If mu and shape are vectors, then rgamma(n=n, shape=shape, scale=mu/shape) will generate random gamma variates of this parameterization, etc.; see GammaDist.

For cqo and cao, taking the logarithm of the response means (approximately) a gaussianff family may be used on the transformed data.

Author(s)

T. W. Yee

References

The parameterization of this VGAM family function is the 2-parameter gamma distribution described in the monograph

McCullagh, P. and Nelder, J. A. (1989) Generalized Linear Models, 2nd ed. London: Chapman & Hall.

See Also

gamma1 for the 1-parameter gamma distribution, gamma2.ab for another parameterization of the 2-parameter gamma distribution, mckaygamma2 for a bivariate gamma distribution, expexp, GammaDist, golf.

Examples

# Essentially a 1-parameter gamma
y = rgamma(n <- 100, shape= exp(1))
fit1 = vglm(y ~ 1, gamma1, trace=TRUE, crit="c")
fit2 = vglm(y ~ 1, gamma2, trace=TRUE, crit="c")
coef(fit1, matrix=TRUE)
Coef(fit1)
coef(fit2, matrix=TRUE)
Coef(fit2)

# Essentially a 2-parameter gamma
y = rgamma(n <- 500, rate=exp(1), shape=exp(2))
fit2 = vglm(y ~ 1, gamma2, trace=TRUE, crit="c")
coef(fit2, matrix=TRUE)
Coef(fit2)
summary(fit2)

[Package VGAM version 0.7-7 Index]