mxCompare {OpenMx}R Documentation

Likelihood ratio test

Description

Compare the fit of a model or set of models to a reference model or set of reference models.

Usage

mxCompare(base, comparison, ..., all = FALSE,
  boot=FALSE, replications=400, previousRun=NULL, checkHess=FALSE)
mxCompareMatrix(models, statistic, ...,
  boot=FALSE, replications=400, previousRun=NULL, checkHess=FALSE)

Arguments

base

A MxModel object or list of MxModel objects.

comparison

A MxModel object or list of MxModel objects.

models

A MxModel object or list of MxModel objects.

statistic

A vector of up to two statistics with which to populate the returned matrix.

...

Not used.

all

A boolean value on whether to compare all bases with all comparisons. Defaults to FALSE.

boot

Whether to use the bootstrap distribution to compute the p-value.

replications

How many replications to use to approximate the bootstrap distribution.

previousRun

Results to re-use from a previous bootstrap.

checkHess

Whether to approximate the Hessian in each replication

Details

The mxCompare function is used to compare the fit of one or more MxMatrix objects with output to one or more comparison models. mxCompareMatrix compares all the models provided against each other. Fit statistics for the comparison model or models are subtracted from the fit statistics for the base model or models. All models included in the ‘base’ argument are also listed without comparison (compared to a <NA> model) to present their raw fit statistics.

Model comparisons are made by subtracting the fit of the comparison model from the fit of a base model. To make sure that the differences between models are positive and yield p-values for likelihood ratio tests, the model or models listed in the ‘base’ argument should be more saturated (i.e., more estimated parameters and fewer degrees of freedom) than models listed in the ‘comparison’ argument. For mxCompareMatrix only the comparisons that make sense will be included. If a comparison is made where the comparison model has a higher minus 2 log likelihood (-2LL) than the base model, then the difference in their -2LLs will be negative. P-values for likelihood ratio tests will not be reported when either the -2LL or degrees of freedom for the comparison are negative.

When multiple models are included in both the ‘base’ and ‘comparison’ arguments, then comparisons are made between the two lists of models based on the value of the ‘all’ argument. If ‘all’ is set to FALSE (default), then the first model in the ‘base’ list is compared to the first model in the ‘comparison’ list, second with second, and so on. If there are an unequal number of ‘base’ and ‘comparison’ models, then the shorter list of models is repeated to match the length of the longer list. For example, comparing base models ‘B1’ and ‘B2’ with comparison models ‘C1’, ‘C2’ and ‘C3’ will yield three comparisons: ‘B1’ with ‘C1’, ‘B2’ with ‘C2’, and ‘B1’ with ‘C3’. Each of those comparisons are prefaced by a comparison between the base model and a missing comparison model to present the fit of the base model.

If ‘all’ is set to TRUE, all possible comparisons between base and comparison models are made, and one entry is made for each base model. All comparisons involving the first model in ‘base’ are made first, followed by all comparisons with the second ‘base’ model, and so on. When there are multiple models in either the ‘base’ or ‘comparison’ arguments but not both, then the ‘all’ argument does not affect the set of comparisons made.

The following columns appear in the output:

base

Name of the base model.

comparison

Name of the comparison model. Is <NA> for the first

ep

Estimated parameters of the comparison model.

minus2LL

Minus 2*log-likelihood of the comparison model. If the comparison model is <NA>, then the minus 2*log-likelihood of the base model is given.

df

Degrees in freedom of the comparison model. If the comparison model is <NA>, then the degrees of freedom of the base model is given.

AIC

Akaike's Information Criterion for the comparison model. If the comparison model is <NA>, then the AIC of the base model is given.

diffLL

Difference in minus 2*log-likelihoods of the base and comparison models. Will be positive when base model -2LL is higher than comparison model -2LL.

diffdf

Difference in degrees of freedoms of the base and comparison models. Will be positive when base model DF is lower than comparison model DF (base model estimated parameters is higher than comparison model estimated parameters)

p

P-value for likelihood ratio test based on diffLL and diffdf values.

The mxCompare function will give a p-value for any comparison in which both ‘diffLL’ and ‘diffdf’ are non-negative. However, this p-value is based on the assumptions of the likelihood ratio test, specifically that the two models being compared are nested. The likelihood ratio test and associated p-values are not valid when the comparison model is not nested in the referenced base model. For a more accurate p-value, the empirical bootstrap distribution can be computed (‘boot=TRUE’). However, ‘replications’ must be set high enough for an accurate approximation. The Monte Carlo SE of a proportion for B replications is √(p*(1-p)/B), but this will be zero if p is zero, which is nonsense. Note that a parametric-bootstrap p-value of zero must be interpreted as p < 1/B, which, depending on B and the desired Type I error rate, may not be "statistically significant."

When ‘boot=TRUE’, the model has a default compute plan, and ‘checkHess’ is kept at FALSE then the Hessian will not be approximated or checked. On the other hand, ‘checkHess’ is TRUE then the Hessian will be approximated by finite differences. This procedure is of some value because it can be informative to check whether the Hessian is positive definite (see mxComputeHessianQuality). However, approximating the Hessian is often costly in terms of CPU time. For bootstrapping, the parameter estimates derived from the resampled data are typically of primary interest.

Use options('digits' = N) to set the minimum number of significant digits to be printed in values. The mxCompare function does not directly accept a digits argument, and depends on the value of the 'digits' option.

See Also

mxModel; options (use options('mxOptions') to see all the OpenMx-specific options)

Examples


data(demoOneFactor)
manifests <- names(demoOneFactor)
latents <- c("G1")
model1 <- mxModel(model="One Factor", type="RAM",
      manifestVars = manifests,
      latentVars = latents,
      mxPath(from = latents, to=manifests),
      mxPath(from = manifests, arrows = 2),
      mxPath(from = latents, arrows = 2, free = FALSE, values = 1.0),
      mxData(cov(demoOneFactor), type = "cov", numObs = 500)
)

fit1 <- mxRun(model1)

latents <- c("G1", "G2")
model2 <- mxModel(model="Two Factor", type="RAM",
      manifestVars = manifests,
      latentVars = latents,
      mxPath(from = latents[1], to=manifests[1:3]),
      mxPath(from = latents[2], to=manifests[4:5]),
      mxPath(from = manifests, arrows = 2),
      mxPath(from = latents, arrows = 2, free = FALSE, values = 1.0),
      mxData(cov(demoOneFactor), type = "cov", numObs=500)
)
fit2 <- mxRun(model2)

mxCompare(fit1, fit2)

# vary precision of the output
oldPrecision = as.numeric(options('digits')) 
options('digits' = 1)
mxCompare(fit1, fit2)
options('digits' = oldPrecision)

[Package OpenMx version 2.8.3 Index]