OpenMx (version 2.22.10)

mxAutoStart: Automatically set starting values for an MxModel

Description

Automatically set starting values for an MxModel

Usage

mxAutoStart(model, type = c("ULS", "DWLS"))

Value

an MxModel with new free parameter values

Arguments

model

The MxModel for which starting values are desired

type

The type of starting values to obtain, currently unweighted or diagonally weighted least squares, ULS or DWLS

Details

This function automatically picks very good starting values for many models (RAM, LISREL, Normal), including multiple group versions of these. It works for models with algebras. Models of continuous, ordinal, and joint ordinal-continuous variables are also acceptable. It works for models with covariance or raw data. However, it does not currently work for models with definition variables, state space models, item factor analysis models, or multilevel models; it only works well for GREML models under certain circumstances (see further below).

The method used to obtain new starting values is quite simple. The user's model is changed to an unweighted least squares (ULS) model. The ULS model is estimated and its final point estimates are returned as the new starting values. Optionally, diagonally weighted least squares (DWLS) can be used instead with the type argument.

Please note that ULS is sensitive to the scales of your variables. For example, if you have variables with means of 20 and variances of 0.001, then ULS will "weight" the means 20,000 times more than the variances and might result in zero variance estimates. Likewise if one variable has a variance of 20 and another has a variance of 0.001, the same problem may arise. To avoid this, make sure your variables are scaled accordingly. You could also use type='DWLS' to have the function use diagonally weighted least squares to obtain starting values. Of course, using diagonally weighted least squares will take much much longer and will usually not provide better starting values than unweighted least squares.

Also note that if model contains a GREML expectation, argument type is ignored, and the function always uses a form of ULS. The function can be very helpful for finding good starting values for GREML MxModels that use an "implicit" (i.e., "specified in terms of covariates via mxExpectationGREML() argument Xvars") model for the phenotypic mean. In contrast, the function is not generally recommended for GREML MxModels that use an "explicit" (i.e., "specified in terms of an MxMatrix or MxAlgebra via argument yhat") model for the phenotypic mean, and indeed, it can be downright counterproductive in such cases.

Examples

# Use the frontpage model with negative variances to show better
# starting values
library(OpenMx)
data(demoOneFactor)

latents  = c("G") # the latent factor
manifests = names(demoOneFactor) # manifest variables to be modeled

m1 <- mxModel("One Factor", type = "RAM",
	manifestVars = manifests, latentVars = latents,
	mxPath(from = latents, to = manifests),
	mxPath(from = manifests, arrows = 2, values=-.2),
	mxPath(from = latents, arrows = 2, free = FALSE, values = 1.0),
	mxPath(from = "one", to = manifests),
	mxData(demoOneFactor, type = "raw")
)

# Starting values imply negative variances!
mxGetExpected(m1, 'covariance')

# Use mxAutoStart to get much better starting values
m1s <- mxAutoStart(m1)
mxGetExpected(m1s, 'covariance')