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Dear SPMers,
I have 2 questions about DCM in SPM8b. Hope that you can help me.
1. How to report the results of comparisons of DCMs?
In SPM5, it is advised to report group Bayesian factors (GBF) to show which model is the best. However, in SPM8b, the model comparisons use the negative free energy (F) as an approximation to the log evidence. In the "BMS.mat", there are 5 variables: BMS.DCM.rfx.SF, BMS.DCM.rfx.F, BMS.DCM.rfx.alpha, BMS.DCM.rfx.exp_r, BMS.DCM.rfx.xp and BMS.DCM.rfx.data. What exactly the first four variables represent? Or is there any paper about them?
2. The results of "average" function of DCM in SPM8b.
After I got the best model after comparisons, I averaged 13 models of subjects and got an averaged DCM model, which contains average parameters and their post probabilities, but not averaged time series. Can I report the average parameters and their post probabilities as my final results?
I also found that the average parameters and post probabilities are different from results of classical statistical analysis outside SPM on the arithmetic (non-Bayesian) means of the subject-specific connection parameters (n=13) with SPSS (Analysis/ Compare means to 0). For example, the average DCM.C is 1.3194 while the mean of DCM.Cs of 13 subjects in SPSS is only 0.3870. I examined all the DCM.Cs of each subject, I found that they were all less than 0.5. What arithmetic in the function of "Average"? And what result is better to be reported?
Thanks a lot for advance.