In the world of SEM, GLS has pretty much fallen by the wayside - I can't recall anything I've seen arguing for it's use in the past 10 years, and I also can't recall anyone using it over ML. The recommendations for non-normal distributions tend to be robust-ML, or robust weighted least squares. These are more computationally intensive, and I *think* that John Fox (author of sem) has written somewhere that it wouldn't be possible to implement them within R, without using a lower level language - or rather that it might be possible, but it would be really, really slow.
However, ML and GLS are pretty similar, if you dug around in the source code, you could probably make the change (see, http://www2.gsu.edu/~mkteer/discrep.html for example, for the equations; in fact GLS is somewhat computationally simpler, as you don't need to invert the implied covariance matrix at each iteration). However, the fact that it's not hard to make the change, and that no one has made the change, is another argument that it's not a change that needs to be made. Jeremy 2009/12/2 Ralf Finne <ralf.fi...@novia.fi>: > Hi R-colleagues. > > I have been using the sem(sem) function. It uses > maximum likelyhood as optimizing. method. > According to simulation study in Umeå Sweden > (http://www.stat.umu.se/kursweb/vt07/stad04mom3/?download=UlfHolmberg.pdf > Sorry it is in swedish, except the abstract) > maximum likelihood is OK for large samples and normal distribution > the SEM-problem should be optimized by GLS (Generalized Least Squares). > > > So to the question: > > Is there any R-function that solves SEM with GLS? > > > Ralf Finne > Novia University of Applied Science > Vasa Finland > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Jeremy Miles Psychology Research Methods Wiki: www.researchmethodsinpsychology.com ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
