[FieldTrip] interaction within-subject indepsamplesT

Eric Maris e.maris at donders.ru.nl
Thu Sep 15 10:10:36 CEST 2011

Dear Kathrin,

> I need your advice on how to correctly test the interaction effects in a
> within-subject design with cluster-based randomization tests. I have two
> factors A and B with the levels a1/a2 and b1/b2. In addition, the number
> trials in A are about twice as much as in B.
> The procedure would be totally clear in a between-subject design (as in a
> previous post on the list:
> http://mailman.science.ru.nl/pipermail/fieldtrip/2011-January/003447.html)

This post is about a WITHIN-subjects design.

> because I could easily compute the differences of the factor levels
> A_diff=a1-a2 and B_diff=b1-b2 and then run ft_freqstatistics on the
> differences. However, since I have to run the statistics across trials and
> subjects I have independent samples and not dependent samples. 

So, you have a between-trials design.

If it is
> possible at all, how can I calculate the differences A_diff and B_diff
> trials? Is it a problem that the number of trials are not equal?

This is not possible. Permutation tests cannot be used for testing
interactions in a multi-factorial between-subjects/trials design. Michael
Wibral has a post in which he refers to a nice statistics paper on the

A proxy that is possible (but not statistically sound) is to perform two
statistical tests of the first factor, once at the first level of second
factor and once at the second level.


Eric Maris


> Any help is appreciated!
> Best,
> Kathrin
> _____________________________________
> Kathrin Müsch
> Dept. of Neurophysiology and Pathophysiology
> University Medical Center Hamburg-Eppendorf
> Martinistr. 52
> 20246 Hamburg
> Germany
> Phone: +49-40-7410-54680
> Fax: +49-40-7410-57752
> E-Mail: k.muesch at uke.uni-hamburg.de
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