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I am in the process of analyzing each run/subject with 6 EVs. However am getting the common rank deficient error. I am assuming this is most probably due to the many contrasts. Is there a way around this. To reiterate my scanning protocol, I have five acquisitions per subject. In essence, it is one stimuli split across the 5 scanning sessions, and I currently have 6 EV. Actually I have more, but 6 will do for now. Thanks ! On Thu, Nov 12, 2009 at 6:03 PM, Jesper Andersson <[log in to unmask]>wrote: > Hi again Dav, > > > The one thing that still seems to be a bit of an issue is whether I can >> correct for multiple contrasts, for example, if I have 1 contrast of >> interest, or 2 or 100 (in theory only!). If I do higher-level contrasts >> independently for just 1 contrast vs. 100, I am nearly guaranteed to get >> spurious significance in the latter case. In my case, I have a handful of >> contrasts (which are actually largely independent - along the lines of >> modelling 3 two-level factors and the interactions between them). Thus, I am >> still a little concerned about correcting for these multiple (but at least >> partially independent) contrasts. >> >> Or is this handled already by those contrasts having been specified >> simultaneously at the first two levels? >> > > this is something that is, funnily enough, largely ignored in neuroimaging. > If you ask a question through some contrast and threshold the resulting SPM > at a FWE-rate (i.e. corrected for multiple comparisons among the voxels in > that SPM) of 0.05 you basically say that you accept 1 false positive out of > every 20 times you test a contrast. > > If you use two different contrasts in the same data the false positive rate > pretty much doubles, for the experiment as a whole. And so it goes as you > keep coming up with more contrasts. > > So you are right that in your average neuroimaging paper the false positive > rate is typically much higher than 0.05, for the paper/study as a whole. > > This is very easy for you to "fix" yourself. Let's say you are doing a > study where you want to test four different contrasts. Test them at a 0.05/4 > FWE level instead, and you will have a false positive rate of 0.05 for your > paper/study. > > Chances are you'll report fewer blobs though ;-) > > Good luck Jesper >