[pymvpa] Balancing strategy

[Chris Johnson]

Hi,

I have an experimental (fMRI) paradigm wherein the conditions were
balanced across runs, and performed a searchlight MVPA analysis with
leave-one-run-out cross validation. In some subjects, however, we found
excessive head motion and had to eliminate scans, which has resulted in
some condition imbalance across runs.

There are two options that occur to me to resolve this, and in each
case, I would use mvpa2.generators.resampling.Balancer.

1) Attempt to reconstruct validation folds of the original size,
possibly reducing the number of folds.
2) Attempt to reconstruct the same number of folds, but with fewer
instances of each condition.

I don't have a principled way to choose between these two options, and
it's possible that I've missed other, better options. I'd appreciate any
insight you might have on the advantages of each strategy and any
considerations that might come into play.

Thanks,
-- 
Chris Johnson

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