[R] sem package and AMOS
adick at uchicago.edu
Mon Feb 2 21:50:26 CET 2009
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.
Anthony Steven Dick, Ph.D.
Human Neuroscience Laboratory
Department of Neurology
The University of Chicago
5841 S. Maryland Ave. MC-2030
Chicago, IL 60637
Email: adick at uchicago.edu
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