r.squaredLR: Likelihood-ratio based pseudo-R-squared

Description Usage Arguments Details Value Note References See Also

Description

Calculate a coefficient of determination based on the likelihood-ratio test (R_LR²).

Usage

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r.squaredLR(x, null = NULL, null.RE = FALSE)

null.fit(x, evaluate = FALSE, RE.keep = FALSE, envir = NULL)

Arguments

x

a fitted model object.

null

a fitted null model. If not provided, null.fit will be used to construct it. null.fit's capabilities are limited to only a few model classes, for others the null model has to be specified manually.

null.RE

logical, should the null model contain random factors? Only used if no null model is given, otherwise omitted, with a warning.

evaluate

if TRUE evaluate the fitted model object else return the call.

RE.keep

if TRUE, the random effects of the original model are included.

envir

the environment in which the null model is to be evaluated, defaults to the environment of the original model's formula.

Details

This statistic is is one of the several proposed pseudo-R²'s for nonlinear regression models. It is based on an improvement from null (intercept only) model to the fitted model, and calculated as

R² = 1 - exp(-2/n * ㏒ℓ(x) - ㏒ℓ(0))

where ㏒ℓ(x) and ㏒ℓ(0) are the log-likelihoods of the fitted and the null model respectively. ML estimates are used if models have been fitted by REstricted ML (by calling logLik with argument REML = FALSE). Note that the null model can include the random factors of the original model, in which case the statistic represents the ‘variance explained’ by fixed effects.

For OLS models the value is consistent with classical R². In some cases (e.g. in logistic regression), the maximum R_LR² is less than one. The modification proposed by Nagelkerke (1991) adjusts the R_LR² to achieve 1 at its maximum: R̅² = R² / max(R²) where max(R²) = 1 - exp(2 / n * ㏒ℓ(0)) .

null.fit tries to guess the null model call, given the provided fitted model object. This would be usually a glm. The function will give an error for an unrecognized class.

Value

r.squaredLR returns a value of R_LR², and the attribute "adj.r.squared" gives the Nagelkerke's modified statistic. Note that this is not the same as nor equivalent to the classical ‘adjusted R squared’.

null.fit returns the fitted null model object (if evaluate = TRUE) or an unevaluated call to fit a null model.

Note

R² is a useful goodness-of-fit measure as it has the interpretation of the proportion of the variance ‘explained’, but it performs poorly in model selection, and is not suitable for use in the same way as the information criterions.

References

Cox, D. R. and Snell, E. J. (1989) The analysis of binary data, 2nd ed. London, Chapman and Hall

Magee, L. (1990) R² measures based on Wald and likelihood ratio joint significance tests. Amer. Stat. 44: 250-253

Nagelkerke, N. J. D. (1991) A note on a general definition of the coefficient of determination. Biometrika 78: 691-692

See Also

summary.lm, r.squaredGLMM


MuMIn documentation built on Jan. 31, 2018, 1:03 a.m.