Hi,
could you help provide some insights as to why running the following script, which uses data(twinData), gives negative estimates for Vc in an ACE model?
https://ibg.colorado.edu/cdrom2020/maes/twinModeling/oneACEvc.R
These are the most relevant lines related to the model:
covA <- mxMatrix( type="Symm", nrow=nv, ncol=nv, free=TRUE, values=svPa, label="VA11", name="VA" ) covC <- mxMatrix( type="Symm", nrow=nv, ncol=nv, free=TRUE, values=svPa, label="VC11", name="VC" ) covE <- mxMatrix( type="Symm", nrow=nv, ncol=nv, free=TRUE, values=svPa, label="VE11", name="VE" ) # Create Algebra for expected Variance/Covariance Matrices in MZ & DZ twins covP <- mxAlgebra( expression= VA+VC+VE, name="V" ) covMZ <- mxAlgebra( expression= VA+VC, name="cMZ" ) covDZ <- mxAlgebra( expression= 0.5%x%VA+ VC, name="cDZ" ) expCovMZ <- mxAlgebra( expression= rbind( cbind(V, cMZ), cbind(t(cMZ), V)), name="expCovMZ" ) expCovDZ <- mxAlgebra( expression= rbind( cbind(V, cDZ), cbind(t(cDZ), V)), name="expCovDZ" )
Running the script provides the following solution:
fitACE$output$confidenceIntervals lbound estimate ubound oneACEvc.US[1,1] 0.60785108 0.75506776 0.9254683654 oneACEvc.US[1,2] -0.29756250 -0.14471244 -0.0061102627 oneACEvc.US[1,3] 0.15055312 0.16935009 0.1913908419
These estimates are further reported on page 71 of the accompanying slides found at
https://ibg.colorado.edu/cdrom2020/maes/twinModeling/TwinModeling2020c.pdf
I don’t really understand why these results would make sense. Of course, just looking at the slides I don’t have the full narrative, which is why I am asking here.
If VA/VC/VE are known to be variances, wouldn't including lbound=0 in the mxMatrix functions be more appropriate? Was that just an oversight? The slides further discuss negative C in the next slide, so it doesn't appear it was.
Furthermore, if setting lbound=0 is appropriate, I noticed that the confidence intervals failed to be calculated:
lbound estimate ubound oneACEvc.US[1,1] 0.54832905 6.1730227e-01 NA oneACEvc.US[1,2] NA 1.3552200e-12 NA oneACEvc.US[1,3] NA 1.7304629e-01 NA
can this be fixed?
Thanks for your help,
Mathieu