sem package and AMOS
Hi John- Thanks. You were right--AMOS was not reading the sample size correctly (i.e., I was not telling it correctly). When I corrected the problem, I got the same estimates. Thanks for pointing me in the right direction. Anthony
John Fox wrote:
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 <adick at 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: adick at uchicago.edu Web: http://home.uchicago.edu/~adick/
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-------------------------------- John Fox, Professor Department of Sociology McMaster University Hamilton, Ontario, Canada http://socserv.mcmaster.ca/jfox/
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: adick at uchicago.edu Web: http://home.uchicago.edu/~adick/