genpoisson {VGAM}R Documentation

Generalized Poisson distribution

Description

Estimation of the two parameters of a generalized Poisson distribution.

Usage

genpoisson(llambda = "logit", ltheta = "loge",
           elambda=list(), etheta=list(),
           ilambda = 0.5, itheta = NULL, zero = NULL)

Arguments

llambda Parameter link function for lambda. See Links for more choices. The parameter lies in the unit interval, therefore the logit link is a reasonable default.
ltheta Parameter link function for theta. See Links for more choices. The parameter is positive, therefore the default is the log link.
elambda, etheta List. Extra argument for each of the links. See earg in Links for general information.
ilambda Optional initial value for lambda.
itheta Optional initial value for theta.
zero An integer vector, containing the value 1 or 2. If so, lambda or theta respectively are modelled as an intercept only. The default value NULL means both linear/additive predictors are modelled as functions of the explanatory variables.

Details

See Consul (1989) for details.

Value

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

Note

Convergence problems may occur when lambda is very close to 0. This distribution is useful for dispersion modelling.

Author(s)

T. W. Yee

References

Jorgensen, B. (1997) The Theory of Dispersion Models. London: Chapman & Hall

Consul, P. C. (1989) Generalized Poisson Distributions: Properties and Applications, Marcel Dekker.

See Also

poissonff.

Examples

y = rpois(n=100, lam=24)
fit  = vglm(y ~ 1, genpoisson, trace=TRUE)
fitted(fit)[1:5]
mean(y)
summary(fit)
coef(fit, matrix=TRUE)
Coef(fit)

[Package VGAM version 0.7-4 Index]