Your FDR analysis sounds correct. You probably have a rather small
number of "marginally" significant vertices, which is why none survive
FDR. You could try increasing the "q" value from say 0.05 to 0.1, in
which case 10% of the surviving vertices would be expected to be false
positives.
cheers,
Mike H.
On Fri, 2009-10-16 at 12:03 +0200, Yulia WORBE wrote:
> Dear Freesurfer team,
>
> We are currently doing a cortical thickness studies between a group of
> psychiatric patients (n=60) and controls (n=30). We tested several
> smoothing levels (15mm, 20mm, 25mm)
>
> When setting an uncorrected threshold (such as p<0.005), we obtained
> several regions of decreased thickness, which are consistent with the
> pathology.
>
> However, when trying to correct for multiple comparisons using FDR
> ("Set Using FDR" button in qdec), the computed threshold is very high
> (e.g. 4.3 for 20mm smoothing) and, obviously, no significant regions
> are left.
>
> Did we do anything wrong in the analysis ?
>
> Thank you very much for your help,
> Yulia
>
>
>
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