[FieldTrip] interactions

[Joshua Hartshorne]

Hi List!

I have seen around a dozen comments in the archives that interactions can't
be tested by permutation for within-subject designs. I haven't been able to
find a thread that explains why not. It seems like in a 2x2 design, you
could still pick one of the conditions and permute the labels. I'm sure
there's a proof somewhere for why this doesn't work, and it would be great
to see it.

Similarly, for the mixed design, why permute the between-subject labels?
Why not permute the within-subject labels instead? Actually, why not do
both? I follow the reasoning why permuting both is overkill, but not why
it's wrong.

If someone could explain, it would be much appreciated. Knowing what to do
is good, but it would be even better to understand why.

Thanks,
Josh
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