gammahyperbola {VGAM} | R Documentation

## Gamma Hyperbola Bivariate Distribution

### Description

Estimate the parameter of a gamma hyperbola bivariate distribution by maximum likelihood estimation.

### Usage

    gammahyperbola(ltheta = "loglink", itheta = NULL, expected = FALSE)

### Arguments

`ltheta` | Link function applied to the (positive) parameter theta. See `Links` for more choices.
`itheta` | Initial value for the parameter. The default is to estimate it internally.
`expected` | Logical. `FALSE` means the Newton-Raphson (using the observed information matrix) algorithm, otherwise the expected information matrix is used (Fisher scoring algorithm).

### Details

The joint probability density function is given by

f(y1,y2) = exp( -exp(-theta) * y1 / theta - theta * y2)

for theta > 0, y1 > 0, y2 > 1. The random variables Y1 and Y2 are independent. The marginal distribution of Y1 is an exponential distribution with rate parameter exp(-theta)/theta. The marginal distribution of Y2 is an exponential distribution that has been shifted to the right by 1 and with rate parameter theta. The fitted values are stored in a two-column matrix with the marginal means, which are theta * exp(theta) and 1 + 1/theta.

The default algorithm is Newton-Raphson because Fisher scoring tends to be much slower for this distribution.

### 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 a two-column matrix.

### Author(s)

T. W. Yee

### References

Reid, N. (2003) Asymptotics and the theory of inference. Annals of Statistics, 31, 1695–1731.

### See Also

`exponential`.

### Examples

    gdata <- data.frame(x2 = runif(nn <- 1000))
    gdata <- transform(gdata, theta = exp(-2 + x2))
    gdata <- transform(gdata, y1 = rexp(nn, rate = exp(-theta)/theta),
                              y2 = rexp(nn, rate = theta) + 1)
    fit <- vglm(cbind(y1, y2) ~ x2, gammahyperbola(expected = TRUE), data = gdata)
    coef(fit, matrix = TRUE)
    Coef(fit)
    head(fitted(fit))
    summary(fit)

[Package VGAM version 1.1-1 Index]