This is an elementary nonurgent question but an answer would help me understand OpenMx better.
In trying to teach myself a bit more I have been playing around with a couple of multivariate ACE twin scripts. There's one by Hermine Maes that I downloaded from:
and another I think by Steve Boker that is in the slides from
They differ in how they specify covariance matrices for the saturated model:
The Maes script gives:
mxMatrix( type="Lower", nrow=ntv, ncol=ntv, free=TRUE, values=pathstartvalue, name="CholMZ" ),
mxAlgebra( expression=CholMZ %*% t(CholMZ), name="ExpCovMZ" ),
mxAlgebra( expression=diag2vec(ExpCovMZ), name="ExpVarMZ"),
The Boker script gives:
mxMatrix( "Diag", ntv, ntv, free=T, values=meanestvalue, lbound=.001, name="EVmzf" ),
mxMatrix( "Stand", ntv, ntv, free=T, values=.5, lb=-.99, ub=.99, name="ERmzf" ),
mxAlgebra( expression= EVmzf %*% ERmzf %*% EVmzf, name="ECmzf" ),
I assume they are mathematically equivalent, with the Boker script estimating the correlation matrix (ERmzf) with the Stand matrix.
My problem is that when I try to run the Boker script, I get:
Error: The job for model 'multi' exited abnormally with the error message: Expected covariance matrix is not positive definite
I understand what the error message is saying – I'm just wondering why you'd want to estimate the covariance matrix this way, if it can give a 'not positive definite' result?
Or maybe this was an example to demonstrate the perils of not using a regular Cholesky decomposition ?
It's possible that the dataset I'm using is not the one it was written for.