[pymvpa] FW: What does a classification accuracy that is significantly lower than chancel level mean?

Yaroslav Halchenko debian at onerussian.com
Fri Nov 9 18:23:35 UTC 2012

>    I'm running MVPA on some fMRI data (four different stimuli, say A, B, C
>    and D; six runs in each subject) to see whether the BOLD signals from a
>    given ROI can successfully predict the type of the stimulus. The MVPA
>    (leave-one-run-out cross-validation) was performed on each subject for
>    each two-way classification task. In a particular classification task (say
>    classification A vs. B), in some subjects, the classification accuracy was
>    (almost) significantly LOWER than the chance level (somewhere between 0.2
>    and 0.4).

depending on number of trials/cross-validation scheme even values of 0
could come up by chance ;-)   but indeed should not be 'significant'

>    What could be the reason for a significantly-lower-than-chance-level
>    accuracy?

and how significant is this 'significantly LOWER'?
details of # trials/cross-validation?

> The P value was obtained from 10,000 permutations. 

is that permutations within the subject which at the end showed
significant below 0? how permuations were done?

> But the
>    accuracies of all other classifications look fine in all subjects.

fine means all above chance or still distributed around chance?

Yaroslav O. Halchenko
Postdoctoral Fellow,   Department of Psychological and Brain Sciences
Dartmouth College, 419 Moore Hall, Hinman Box 6207, Hanover, NH 03755
Phone: +1 (603) 646-9834                       Fax: +1 (603) 646-1419
WWW:   http://www.linkedin.com/in/yarik        

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