Extract AIC from a Fitted Model

Usage

extractAIC    (fit, scale,     k = 2, ...)  
extractAIC.lm (fit, scale = 0, k = 2, ...)
extractAIC.glm(fit, scale = 0, k = 2, ...)  
extractAIC.aov(fit, scale = 0, k = 2, ...)  
extractAIC.coxph  (fit, scale, k = 2, ...)  
extractAIC.negbin (fit, scale, k = 2, ...)  
extractAIC.survreg(fit, scale, k = 2, ...)

Arguments

fit fitted model, usually the result of a fitter like lm.
scale optional numeric specifying the scale parameter of the model, see scale in step.
k numeric specifying the ``weight'' of the equivalent degrees of freedom (=:edf) part in the AIC formula.
... further arguments (currently unused in base R).

Description

Computes the (generalized) Akaike Information Criterion for fit, i.e.

AIC = - 2*log L + k * edf,

where L is the likelihood and edf the equivalent degrees of freedom (i.e., the number of parameters for usual parametric models) of fit.

Details

For generalized linear models (i.e., for lm, aov, and glm), -2log L is the deviance, as computed by deviance(fit).

k = 2 corresponds to the traditional AIC, using k = log(n) provides the BIC (Bayes IC) instead.

For further information, particularly about scale, see step.

Value

A numeric vector of length 2, giving
edf the ``equivalent degrees of freedom'' of the fitted model fit.
AIC the (generalized) Akaike Information Criterion for fit.

Note

These functions are used in add1, drop1 and step and that may be their main use.

Author(s)

B. D. Ripley

References

Venables, W. N. and B. D. Ripley (1997). Modern Applied Statistics with S-Plus. New York: Springer (2nd ed).

See Also

deviance, add1, step

Examples

example(glm)
extractAIC(glm.D93)#>>  5  15.129

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