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Dear Freesurfer Experts:
I analyzed my data following the guidelines on the FSL Tutorial/Group Analysis page . I am trying to plot the adjusted values (covariates: age, sex, and ICV) for the significant clusters for each of my participants.
I set up the contrast to test group differences. So testing whether there is a difference between groups regressing out the effects of sex, age, and ICV. Contrasts created were [1 -1 0 0 0 0 0 0] and [-1 1 0 0 0 0 0 0].
I then extracted values from this file (cache.th20.abs.sig.y.ocn.dat; i.e., the average value of each subject in each cluster) into my dataset. I believe these are the unadjusted values, correct? I calculated total volume and total surface area by multiplying these values by the total number of vertices in each cluster. To adjust for covariates, I ran a regression with the ExtractedClusterValues as the DV and Group, Sex, Age, and ICV as predictors. Looking at the output, there was no significant effect of group (p = .769; whereas in FreeSurfer the cluster-wise p < .001). Also when I plotted the predicted values adjusting for covariates, the means between the groups were very similar.
Why am I getting a p = .769 when I adjust for covariates, but FreeSurfer has a cluster-wise p < .001? Or is there a way to calculate adjusted values in FreeSurfer?
Best,
Jessica