[R] sem package and AMOS

[Anthony Dick]

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/