[R] SEM multigroup modeling

[John Fox]

Dear Maya,

You and I corresponded about this problem yesterday, so I'll respond on the r-help list only briefly:

On Tue, 23 Oct 2012 23:12:23 +0300
 Maya Abou Zeid <ma202 at aub.edu.lb> wrote:
> Hello,
> 
> I am using the SEM package in R to fit a multigroup latent variable model and ran into some difficulties. I have 2 questions:
> 
> 
> 1.       First, I am getting the following error message and wondering what to do to fix it:
> 
> Error in solve.default((N[g] - 1) * robustVcov(mod.g, adj.obj = adj.objects[[g]])) :
> 
>   system is computationally singular: reciprocal condition number = 4.52055e-23

It's not really possible to try to locate the source of the problem without the data. You could try several things to get a proper solution, as mentioned in ?sem, and you might try suppressing the computation of the robust coefficient covariance matrix, which is where the problem surfaces.

> 
> 
> 
> 2.       Second, I want to specify different models for two groups (Drivers vs. Nondrivers), but I want only 1 parameter to be different, while everything else remains the same. Should I just create two files with the model equations? But then how do I use these 2 files as an input to the multigroupModel or sem functions? I tried the following but got the error message {Error in model[, 2] : incorrect number of dimensions}. Here is a sample of the code:
> 
> 
> mod.QT.1 <- specifyEquations(...) # model equations for category 1
> 
> mod.QT.2 <- specifyEquations(...) # model equations for category 2
> 
> Lst.model <- list(mod.QT.1, mod.QT.2)
> 
> 
> 
> mod.mg <- multigroupModel(Lst.model, groups=c("Driver", "Nondriver"))
> 
> 
> 
> sem.QT <- sem(mod.mg, data=QTrav, group="Driver_Class1", formula = ~ ..., fixed.x=c(...))

multigroupModel() doesn't take a list of models as an argument but rather arbitrarily named iniital arguments, one for each group. See ?multigroupModel for (some) more information.

> 
> Are there any sample codes for multigroup modeling using the SEM package that I can look at?

There are a couple of examples in ?sem, but I know you've already seen them. These are given in a little more detail in <http://socserv.socsci.mcmaster.ca/jfox/Books/Companion/appendix/Appendix-SEMs.pdf>.

Best,
 John

------------------------------------------------
John Fox
Sen. William McMaster Prof. of Social Statistics
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/
	
> 
> Thanks in advance,
> Maya
> 
> 
> 
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> 
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