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

[Meng Liang]

Hi

I'm not sure if my previous email was sent to the list successfully so I'm sending it again (see below). Could anyone help?

Thanks!
Meng

From: meng.liang at hotmail.co.uk
To: pkg-exppsy-pymvpa at lists.alioth.debian.org
Subject: What does a classification accuracy that is significantly lower than chancel level mean?
Date: Fri, 26 Oct 2012 21:24:57 +0100





Dear experts,

Is it reasonable to get a classification accuracy that is significantly LOWER than the chance level?

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). The P value was obtained from 10,000 permutations. But the accuracies of all other classifications look fine in all subjects.

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

Any ideas?

Many thanks!
Meng


 		 	   		   		 	   		  
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