| gamma.shape {MASS} | R Documentation |
Estimate the Shape Parameter of the Gamma Distribution in a GLM Fit
Description
Find the maximum likelihood estimate of the shape parameter of
the gamma distribution after fitting a Gamma generalized
linear model.
Usage
gamma.shape(object, ...)
## S3 method for class 'glm'
gamma.shape(object, it.lim = 10,
eps.max = .Machine$double.eps^0.25, verbose = FALSE, ...)
Arguments
object |
Fitted model object from a |
it.lim |
Upper limit on the number of iterations. |
eps.max |
Maximum discrepancy between approximations for the iteration process to continue. |
verbose |
If |
... |
further arguments passed to or from other methods. |
Details
A glm fit for a Gamma family correctly calculates the maximum likelihood estimate of the mean parameters but provides only a crude estimate of the dispersion parameter. This function takes the results of the glm fit and solves the maximum likelihood equation for the reciprocal of the dispersion parameter, which is usually called the shape (or exponent) parameter.
Value
List of two components
alpha |
the maximum likelihood estimate |
SE |
the approximate standard error, the square-root of the reciprocal of the observed information. |
References
Venables, W. N. and Ripley, B. D. (2002) Modern Applied Statistics with S. Fourth edition. Springer.
See Also
Examples
clotting <- data.frame(
u = c(5,10,15,20,30,40,60,80,100),
lot1 = c(118,58,42,35,27,25,21,19,18),
lot2 = c(69,35,26,21,18,16,13,12,12))
clot1 <- glm(lot1 ~ log(u), data = clotting, family = Gamma)
gamma.shape(clot1)
gm <- glm(Days + 0.1 ~ Age*Eth*Sex*Lrn,
quasi(link=log, variance="mu^2"), quine,
start = c(3, rep(0,31)))
gamma.shape(gm, verbose = TRUE)
## IGNORE_RDIFF_BEGIN
summary(gm, dispersion = gamma.dispersion(gm)) # better summary
## IGNORE_RDIFF_END