Regardless: FDR's sensitivity appears resolution-dependent to me. On 10/16/2009 10:39 AM, Michael Harms wrote: > Interesting post Donna, but my understanding of FDR is that it sets the > p-value threshold based on the LARGEST p-value that satisfies the FDR > relationship. > > That is, steps 3 and 4 in Genovese et al. (2002) are: > 3) Let r be the largest i for which p <= i/V*q (assuming c=1) > 4) Threshold the image at the p-value p(r). > > So, it isn't the case that you require the most significant p-value to > satisfy p <= 0.05/V "just to get past i=1" as you put it in your post. > > Rather, you pick the largest p-value that satisfies the relationship, > meaning that lower (more-significant) p-values may not have necessarily > satisfied p <= i/V*q for their particular position in the sorted list of > p-values. > > cheers, > Mike H. > > > On Fri, 2009-10-16 at 10:13 -0500, Donna Dierker wrote: > >> I never heard anything on my post here, but it might just be high >> surface resolution: >> >> http://www.mail-archive.com/neuro-mult-c...@brainvis.wustl.edu/msg00026.html >> >> On 10/16/2009 09:58 AM, Michael Harms wrote: >> >>> 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 >>>> >>>> >>>> >>>> _______________________________________________ >>>> Freesurfer mailing list >>>> Freesurfer@nmr.mgh.harvard.edu >>>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer >>>> >>>> >>> _______________________________________________ >>> Freesurfer mailing list >>> Freesurfer@nmr.mgh.harvard.edu >>> https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer >>> >>> > > _______________________________________________ > Freesurfer mailing list > Freesurfer@nmr.mgh.harvard.edu > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/freesurfer >
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