[pymvpa] Update on: Suspicious results

[MS Al-Rawi]

> - 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 <nynkevanderlaan at gmail.com>
> To: pkg-exppsy-pymvpa at lists.alioth.debian.org
> 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
> 
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