[R] sem package and AMOS

[Anthony Dick]

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/