R: Log-Likelihood of a loglm Object

logLik.loglm {vcdExtra} | R Documentation

## Log-Likelihood of a loglm Object

### Description

Calculates the log-likelihood value of the `loglm` model represented by `object` evaluated at the estimated coefficients.

It allows the use of `AIC` and `BIC`, which require that a `logLik` method exists to extract the corresponding log-likelihood for the model.

### Usage

    ## S3 method for class 'loglm'
    logLik(object, ..., zero=1E-10)

### Arguments

`object` | A `loglm` object
`...` | For compatibility with the S3 generic; not used here
`zero` | value used to replace zero frequencies in calculating the log-likelihood

### Details

If cell frequencies have not been stored with the `loglm` object (via the argument `keep.frequencies = TRUE`), they are obtained using `update`.

This function calculates the log-likelihood in a way that allows for non-integer frequencies, such as the case where 0.5 has been added to all cell frequencies to allow for sampling zeros. If the frequencies still contain zero values, those are replaced by the value of `start`.

For integer frequencies, it gives the same result as the corresponding model fit using `glm`, whereas `glm` returns `-Inf` if there are any non-integer frequencies.

### Value

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

### Author(s)

Achim Zeileis

### See Also

`loglm`, `AIC`, `BIC`,

### Examples

    data(Titanic, package="datasets")

    require(MASS)
    titanic.mod1 <- loglm(~ (Class * Age * Sex) + Survived, data=Titanic)
    titanic.mod2 <- loglm(~ (Class * Age * Sex) + Survived*(Class + Age + Sex), data=Titanic)
    titanic.mod3 <- loglm(~ (Class * Age * Sex) + Survived*(Class + Age * Sex), data=Titanic)

    logLik(titanic.mod1)
    AIC(titanic.mod1, titanic.mod2, titanic.mod3)
    BIC(titanic.mod1, titanic.mod2, titanic.mod3)

    # compare with models fit using glm()
    titanic <- as.data.frame(Titanic)
    titanic.glm1 <- glm(Freq ~ (Class * Age * Sex) + Survived,
                        data=titanic, family=poisson)
    titanic.glm2 <- glm(Freq ~ (Class * Age * Sex) + Survived*(Class + Age + Sex),
                        data=titanic, family=poisson)
    titanic.glm3 <- glm(Freq ~ (Class * Age * Sex) + Survived*(Class + Age * Sex),
                        data=titanic, family=poisson)

    logLik(titanic.glm1)
    AIC(titanic.glm1, titanic.glm2, titanic.glm3)
    BIC(titanic.glm1, titanic.glm2, titanic.glm3)

[Package vcdExtra version 0.7-1 Index]