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oh and for FE z>2.3 and p<.05 On Thu, Nov 26, 2009 at 11:46 AM, Shal Hat <[log in to unmask]> wrote: > So, this is what I did: > > 1) First level analysis for each of the 5 runs for each subject with z>2.3 > and p<.0125 (.05/4 contrasts) > 2) Higher level FE analysis of the 5 runs for each subject. > 3) I haven't done this, but plan to: FLAME ME for all subjects. > > Up to number 2 above, I am not seeing any activation. My question is, am I > being too conservative with my z > and p-value thresholds? Any suggestions regarding these values in this > context? > > Thanks !!! > > > > On Sun, Nov 22, 2009 at 2:17 PM, Shal Hat <[log in to unmask]> wrote: > >> I'm pretty sure I haven't. However, from what I read, I understood that >> the SVD algorithm used is very conservative. >> Also,it is worth noting that the diagonal on the matrix has a few dark >> squares (0.000). >> >> >> >> On Sun, Nov 22, 2009 at 11:12 AM, David V. Smith <[log in to unmask]>wrote: >> >>> I don't think that error has anything to do with your contrasts. Your EVs >>> must be highly correlated. Perhaps you mistakenly entered the same EV twice? >>> >>> >>> On Nov 20, 2009, at 5:24 PM, Shal Hat wrote: >>> >>> 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 >>>> >>> >>> >>> >> >