Dear Anthony, sem() does FIML estimation, not 2SLS, and so it's hard to understand why you're getting "nearly identical" parameter estimates but very different coefficient standard errors and model chi-squares. In fact, unless the problem is very ill-conditioned, the parameter estimates should be the same within rounding error, as should the model chi-square. There is some room for small differences in the standard errors -- sem() uses a numerical Hessian and I'm not sure what AMOS does -- but large differences are indicative of some problem.
I suspect that you're not fitting quite the same model in sem() and AMOS. I hope this helps, John On Mon, 02 Feb 2009 14:50:26 -0600 Anthony Dick <ad...@uchicago.edu> wrote: > Hello- > > I am using R to build my initial models, but need to use AMOS to > compare the models of two groups (adults vs. kids). The problem is I > am getting different results with R and AMOS for the initial models > of the separate groups (and the R results make more sense). > > The parameter estimates (path coefficients and variances) from both > programs are nearly identical, but the model chi-squares (and > significance estimates of the parameter estimates) are different. I > am using Maximum Likelihood in AMOS. R I think defaults to > two-stage-least squares estimate, and AMOS 16 does not implement > 2SLS. > > I am using fMRI data, so the error variances are likely correlated, > and the data non-normal to varying degrees. Is 2SLS the better way to > go for these kinds of data? > > Is there a way to change the default method for R sem? I couldn't > find this in the ?help. I note I have run some of the AMOS examples > in R and have gotten identical results across platforms, so I believe > the problem is not in specifying things incorrectly across platforms. > Also, the dfs are identical for both analyses. I must use AMOS to do > model comparison (and thus maximum likelihood), but would like to > achieve similar results across platforms for the basic models before > I proceed, and would like to track down the reason for the > difference. > > Thanks, > > Anthony > > -- > Anthony Steven Dick, Ph.D. > Post-Doctoral Fellow > Human Neuroscience Laboratory > Department of Neurology > The University of Chicago > 5841 S. Maryland Ave. MC-2030 > Chicago, IL 60637 > Phone: (773)-834-7770 > Email: ad...@uchicago.edu > Web: http://home.uchicago.edu/~adick/ > > ______________________________________________ > 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. -------------------------------- John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada http://socserv.mcmaster.ca/jfox/ ______________________________________________ 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.
