Dear all,
I apologize if the question has been previously answered.
We have 2 groups A and B, that we sub-divised in 2 sub-groups 1 and 2. We therefore tried to compare them using a full factorial design entered as A1, A2, B1, B2.
First question: is it the best way to proceed?
We specifically wanted to look at the interaction between group and sub-group, and therefore looked at the F-test. However, using SPM8, we cannot correct our results at the cluster level.
Second question: how can we report these results, if not corrected? Just by mentioning them but not really discuss them? What is the (statistical) validity of uncorrected results on a F-test?
When looking at the corresponding t-tests, we could therefore correct our maps for multiple comparisons.
Third question: Even if we correct these t-tests at the cluster-level, using the same initial threshold that the F-test (say p<0.005 uncorr), is there any other (additional) correction to add (ie by dividing the initial threshold by the number of test AND multiple comparison at the cluster level)? Or is this too conservative? What would be the best method to present the results with sufficient correction?
The first t-test, included in SPM contrast manager by default [1 -1 -1 1]), did not give us any surviving cluster (just with cluster level correction). However, we created the "reverse" contrast ([-1 1 1-1]; still cluster-corrected), that gave us a lot of information!
Fourth question: we tentatively interpreted this as "there was greater difference between B1 and B2, than in A1 and A2. Is that right? If not, what is the meaning of such interaction, and what means the "direction" of the contrast (ie [1 -1 -1 1] or [-1 1 1 -1])?
Thanks for your help!
Best,
Yann
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