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Mon, 03/25/2019 - 15:16
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bwiernik [5]
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Change in type="cor" behavior?

In OpenMx v. 2.11.5, the fit statistics output is the same whether type="cov" or type="cor". In a previous version of OpenMx, I recall that the manifest variable variances would be subtracted from the number of degrees of freedom and observed statistics, and the fit statistics were calculated using the reduced degrees of freedom. Now, the number of observed statistics number is reduced, but the degrees of freedom and fit statistics are not similarly adjusted.

Also, previously the warning about standard errors and fit statistics for type="cor" being potentially incorrect with the Steiger (1980) citation was printed when summary.MxModel() was called as when mxRun() was called. Now the warning only prints with mxRun()

For example, using the HS.ability.data example in help(mxModel), the output for type="cov" is:

Model Statistics: 
               |  Parameters  |  Degrees of Freedom  |  Fit (-2lnL units)
       Model:             21                     24              10970.90
   Saturated:             45                      0              10928.42
Independence:              9                     36              11882.01
Number of observations/statistics: 301/45
 
chi-square:  χ² ( df=24 ) = 42.48505,  p = 0.01138111
Information Criteria: 
      |  df Penalty  |  Parameters Penalty  |  Sample-Size Adjusted
AIC:      -5.514954               84.48505                 87.79687
BIC:     -94.485600              162.33436                 95.73430
CFI: 0.9798547 
TLI: 0.9697821   (also known as NNFI) 
RMSEA:  0.05058496  [95% CI (0.01638508, 0.07931089)]
Prob(RMSEA <= 0.05): 0.4530891

The output for type="cor" is:

Model Statistics: 
               |  Parameters  |  Degrees of Freedom  |  Fit (-2lnL units)
       Model:             21                     15              1788.898
   Saturated:             45                     -9              1746.413
Independence:              9                     27              2700.000
Number of observations/statistics: 301/36
 
chi-square:  χ² ( df=24 ) = 42.48505,  p = 0.01138111
Information Criteria: 
      |  df Penalty  |  Parameters Penalty  |  Sample-Size Adjusted
AIC:       12.48505               84.48505                 87.79687
BIC:      -43.12161              162.33436                 95.73430
CFI: 0.9798547 
TLI: 0.9697821   (also known as NNFI) 
RMSEA:  0.05058496  [95% CI (0.01638508, 0.07931089)]
Prob(RMSEA <= 0.05): 0.4530891

I'm not sure which version this behavior changed in.

I'm not sure when the change occurred. Was this an intentional change? If so, was it determined that the unadjusted df/fit statistics are more likely to be correct (the problems with doing covariance structure analyses with correlations acknowledged)?

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