Mplus/amos degrees of freedom discrep... PreviousNext
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 Mike C Parent posted on Sunday, January 11, 2009 - 4:54 pm
Hello,
I'm running two models in Mplus, one with a variable functioning as a mediator and one without. The mediation model looks like:

v6 ON v5;
v5 ON v4 v2;
v4 ON v3 v1;
v3 ON v2 v1;
v2 ON v1;

The non-mediation model looks like:

v6 ON v5;
v5 ON v4 v2;
v3 ON v1;
v4 ON v1;
v2 ON v1;

So, v3 no longer mediates between v1 & v2, and v4. It's still a predicted outcome of v1, but has no association with v2 or v4. All analysis properties are exactly the same between the two models.

The original model had 7 df. Since I deleted two paths, should I not have 9 df now? I was reading elsewhere on the forum about differences between Mplus df calculation and some other software, but wanted to make sure I wasn't making a different error (misunderstanding the model construction, meeting a limitation of the demo version, etc.). If this is a calculation difference, is there a reference that I might use?

Thank you very much,
Mike
 Linda K. Muthen posted on Sunday, January 11, 2009 - 5:58 pm
You should obtain the same degrees of freedom with Mplus and Amos or any other software if you have the same model and the same estimator. Check to be certain that the same parameters are being estimated. It may be that there is a default you are not taking into account.
 Mike C Parent posted on Sunday, January 11, 2009 - 7:34 pm
Thank you for your prompt reply. However, I reviewed my input file and am still racking my brain trying to figure out why I'm getting df = 8 for the second model. The first model,

SU_INTENTIONS ON SU_CONFIDENCE;
SU_CONFIDENCE ON DMS OBCS_SURV;
DMS ON OBCS_BS SATAQ_INT;
OBCS_BS ON OBCS_SURV SATAQ_INT;
OBCS_SURV ON SATAQ_INT;

runs perfectly, and gives df = 7. To make sure I hadn't mistyped something, I edited that file to

SU_INTENTIONS ON SU_CONFIDENCE;
SU_CONFIDENCE ON DMS OBCS_SURV;
DMS ON SATAQ_INT;
OBCS_BS ON SATAQ_INT;
OBCS_SURV ON SATAQ_INT;

to eliminate OBCS_BS as a mediator, using the same data file, without making any other alterations to the input. Even though I deleted two paths (in lines 3 and 4), Mplus still outputs df = 8.

I'm not sure if my edit activated some other default. I appreciate any feedback you are able to give on this issue.
 Linda K. Muthen posted on Sunday, January 11, 2009 - 7:40 pm
Please send the files and your license number to support@statmodel.com.
 Linda K. Muthen posted on Sunday, January 11, 2009 - 9:40 pm
There is one parameter in the Mplus model that is not in the Amos model. This is the residual covariance between su_int and obcs_bs. This is the default in Mplus. This is why the degrees of freedom are not as you expected.
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