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Hi,
It seems that OpenMx uses the same formula of RMSEA for both single- and multiple-group analyses. According to Steiger (1998), the RMSEA should be adjusted by a factor of sqrt(K) where K is the no. of groups.
The attached PDF includes the test suggested by Steiger (1998, p. 417):
1. Construct two identical arbitrary data sets (random numbers will suffice).
2. Test one sample with a simple model, for example, a single factor model, and record the RMSEA value.
3. Construct a two-sample model, where each group is tested with the same model used in Step 2, but with no parameters constrained to be equal across populations.
4. The two-sample RMSEA reported by the software should of course be identical to the one-sample value, because two (in theory) completely independent samples have yielded identical fit to two independent versions of the same model. If it is not, check whether multiplying the value by sqrt(2) yields an identical value. If it does, then it is highly likely that the software is generating incorrect values by using the single-sample formula inappropriately.
5. If the test in Step 4 confirms that an error is present, multiply all point estimates and interval estimates of the RMSEA by sqrt(2) to obtain correct values.
Steiger, J. H. (1998). A note on multiple sample extensions of the RMSEA fit index. Structural Equation Modeling: A Multidisciplinary Journal, 5(4), 411–419. http://doi.org/10.1080/10705519809540115
Best,
Mike