Independence of trials in non-parametric testing
e.maris at DONDERS.RU.NL
Tue May 5 13:43:40 CEST 2009
> I have a question regarding independence of trials in non-parametric
> The experimental design that I have is as follows, there are three
> conditions C1, C2 and C3. Each condition is of 1 min duration (e.g.
> rest with eyes open for 1 min).
> Now I am planing on dividing each condition into sixty 1sec fake
> trials and then compare Time-Freq representation of C2-C1 and C3-C1
> etc. for significance.
> But I am not sure whether this style is statistically correct, since
> the trials are fake and I am not sure if I can call them independent
> of each other. I can theoretically count them uncorrelated (?) if I
> separate these fake trials by say 300msec or something.
> Just want to make sure I am going in right direction. Please let me
Your intuition is correct. However, time-freq representations do not make
sense here. You will have to go for freq representations (i.e., averaging
spectral energy over time).
Proving that the fake trials are uncorrelated will not be possible. You will
have settle for plausibility. However, the same holds to some extent for an
experiment with real trials.
So, chopping long trials in pieces is in principle OK from the perspective
of permutation inference.
dr. Eric Maris
Donders Institute for Brain, Cognition and Behavior
Center for Cognition and F.C. Donders Center for Cognitive Neuroimaging
P.O. Box 9104
6500 HE Nijmegen
T:+31 24 3612651
F:+31 24 3616066
E: e.maris at donders.ru.nl
MSc Cognitive Neuroscience: www.ru.nl/master/cns/
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The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.
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