Hello all.
Forgive me if this is just my statistics ignorance speaking, but I have a question that I am having a hard time answering.
I am comparing two groups and then extracting eigenvariates using the VOI tool in SPM5. However, I am getting highly conflicting results. If I compare the two groups and say in the design specification that the variances are EQUAL I get the approximately the same results in the graphics window as I do if I declare the variances UNEQUAL (I am assuming that EQAUL means no correction for non-sphericity and UNEQUAL corrects for non-sphericity). However, if I extract the 1st eigenvariate for a cluster for both cases I get completely different results. For EQUAL variance I get values that corroborate the results I see in the graphics window, however for UNEQUAL variance I get the opposite. To better explain if I take the eigenvariates and do a 2 sample TTEST outside of SPM comparing the two groups: for EQUAL I get group1>group2, but for UNEQUAL I get group2>group1. However, as I have said, the SPM results remain virtually unchanged between the two. I am assuming what I am observing has to do with the non-sphericity correction, but I am unsure how. If anyone could give me an explanation I would be most appreciative.
Thanks for the help.
-John West
IU School of Medicine
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