[pymvpa] high prediction rate in a permutation test

[Yaroslav Halchenko]

On Wed, 18 May 2011, J.A. Etzel wrote:
> But would this give you enough permutations for a decent
> distribution? I usually like at least 1000 if possible, but there
> are usually only a handful of runs.

6! = 720, so you should be quite ok ;)

> Isn't what you're describing
> (run-label copying) a special case of permuting the labels within
> each run? 

yes in the sense that indeed by chance for some permutations you might
get an order which would match some run's order, but it would not be
very probable (depending on number of trials in a run of cause).

> If the runs have the same number of trials one of the
> possible orderings would be the one from the previous run.

which could also happen with permutation within the run as well, just
less probable.

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