R: Extract Log-Likelihood

logLik {stats} | R Documentation

## Extract Log-Likelihood

### Description

This function is generic; method functions can be written to handle specific classes of objects. Classes which have methods for this function include: `"glm"`, `"lm"`, `"nls"` and `"Arima"`. Packages contain methods for other classes, such as `"fitdistr"`, `"negbin"` and `"polr"` in package MASS, `"multinom"` in package nnet and `"gls"`, `"gnls"` `"lme"` and others in package nlme.

### Usage

    logLik(object, ...)

    ## S3 method for class 'lm'
    logLik(object, REML = FALSE, ...)

### Arguments

`object` | any object from which a log-likelihood value, or a contribution to a log-likelihood value, can be extracted.
`...` | some methods for this generic function require additional arguments.
`REML` | an optional logical value. If `TRUE` the restricted log-likelihood is returned, else, if `FALSE`, the log-likelihood is returned. Defaults to `FALSE`.

### Details

`logLik` is most commonly used for a model fitted by maximum likelihood, and some uses, e.g. by `AIC`, assume this. So care is needed where other fit criteria have been used, for example REML (the default for `"lme"`).

For a `"glm"` fit the `family` does not have to specify how to calculate the log-likelihood, so this is based on using the family's `aic()` function to compute the AIC. For the `gaussian`, `Gamma` and `inverse.gaussian` families it assumed that the dispersion of the GLM is estimated and has been counted as a parameter in the AIC value, and for all other families it is assumed that the dispersion is known. Note that this procedure does not give the maximized likelihood for `"glm"` fits from the Gamma and inverse gaussian families, as the estimate of dispersion used is not the MLE.

For `"lm"` fits it is assumed that the scale has been estimated (by maximum likelihood or REML), and all the constants in the log-likelihood are included. That method is only applicable to single-response fits.

### Value

Returns an object of class `logLik`. This is a number with at least one attribute, `"df"` (degrees of freedom), giving the number of (estimated) parameters in the model.

There is a simple `print` method for `"logLik"` objects.

There may be other attributes depending on the method used: see the appropriate documentation. One that is used by several methods is `"nobs"`, the number of observations used in estimation (after the restrictions if `REML = TRUE`).

### Author(s)

José Pinheiro and Douglas Bates

### References

For `logLik.lm`:

Harville, D.A. (1974). Bayesian inference for variance components using only error contrasts. Biometrika, 61, 383–385.

### See Also

`logLik.gls`, `logLik.lme`, in package nlme, etc.

`AIC`

### Examples

    x <- 1:5
    lmx <- lm(x ~ 1)
    logLik(lmx) # using print.logLik() method
    utils::str(logLik(lmx))

    ## lm method
    (fm1 <- lm(rating ~ ., data = attitude))
    logLik(fm1)
    logLik(fm1, REML = TRUE)

    utils::data(Orthodont, package = "nlme")
    fm1 <- lm(distance ~ Sex * age, Orthodont)
    logLik(fm1)
    logLik(fm1, REML = TRUE)

[Package stats version 3.1.2 Index]