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

[Yaroslav Halchenko]

>    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