> - I have randomized the labels of the two categories. This resulted in > the same distribution of accuracies as with the correct labeling (peak > of histogram at 0.6 accuracy)..... Does this mean that there is > contamination across chunks???
(same distribution of accuracies) This might indicate peeking. (0.6 accuracy) You still need to find p-value. Rawi ----- Original Message ---- > From: Nynke van der Laan <[email protected]> > To: [email protected] > Sent: Tue, March 1, 2011 3:03:38 PM > Subject: [pymvpa] Update on: Suspicious results > > Hello all, > > thanks for all the very useful suggestions, it's nice to have so many > people thinking with me! > I have tried a few suggestions with for each returning the same > results (approximately): > - I have tried a linearNuSVMC, this gives approximately the same results. > - I tried a Odd/even splitter instead of a NFold splitter --> approx > the same results > - Detrending the data --> approximately the same results. > - I have randomized the labels of the two categories. This resulted in > the same distribution of accuracies as with the correct labeling (peak > of histogram at 0.6 accuracy)..... Does this mean that there is > contamination across chunks??? > > The design of the fMRI task is as follows: The task exists of 38 > trials = 38 chunks > One trial consists of the following sequence of events: > - 4 sec category 1 > - 2 sec fixationcross > - 4 sec category 2 > - 2 sec fixationcross > - 4 sec other event (of no importance) > - random inter trial interval between 2 and 12 sec > > Thus total trial duration is between 18 and 28 seconds. > > So, in each trial/chunk both category events are presented once. The > order of category 1/2 in the trial is randomized. So in some trials > first 1, then 2 or viceversa. Onsets of events category 1 and 2 are > thus 6 seconds apart, but order is randomized so I would not expect > problems. Between chunks is also enough time I would expect... > > Indeed, I have used blockaveraging. For this the functional scans > between (approx) 3.6 and 6 seconds after onset of the event are > averaged. TR is 0.61 secs. > > Does anyone have any additional suggestions? > > Thanks in advance! > > Best regards, > Nynke > > _______________________________________________ > Pkg-ExpPsy-PyMVPA mailing list > [email protected] > http://lists.alioth.debian.org/mailman/listinfo/pkg-exppsy-pymvpa > _______________________________________________ Pkg-ExpPsy-PyMVPA mailing list [email protected] http://lists.alioth.debian.org/mailman/listinfo/pkg-exppsy-pymvpa
