Ranting on MVPA
The use of MVPA for signal detection/localization in neuroimaging has troubled me for a long time. Somehow the community refuses to acknowledge that for the purpose of localization, multivariate tests (e.g. Hotelling’s ) are preferable. Why are multivariate tests preferable than accuracy tests?
- They are more powerful.
- They are easier to interpret.
- They are easier to implement.
- Because they are not cross validated then:
- They are computationally faster.
- They do not suffer biases in the cross validation scheme.
I read and referee papers where authors go to great lengths to interpret their “funky” results. To them I say: Your cross validation scheme is biased and your test statistic is leaving power on the table! Please consult a statistician and replace your MVPA with a multivariate test. For a more “scientific explanation” read  and .
If you justify the use of the prediction accuracy because it is also an effect-size, then please acknowledge that effect size is a different problem than localization and read the multivariate effect size literature (e.g. ).
When would I really want to use the prediction accuracy as a test statistic? When doing actual decoding and not localization, such as brain-computer interfaces.
 Rosenblatt, Jonathan, Roee Gilron, and Roy Mukamel. “Better-Than-Chance Classification for Signal Detection.” arXiv preprint arXiv:1608.08873 (2016).
 Gilron, Roee, et al. “What’s in a pattern? Examining the type of signal multivariate analysis uncovers at the group level.” NeuroImage 146 (2017): 113-120.
 Olejnik, Stephen, and James Algina. “Measures of effect size for comparative studies: Applications, interpretations, and limitations.” Contemporary educational psychology 25.3 (2000): 241-286.