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.li...@hotmail.co.uk
To: pkg-exppsy-pymvpa@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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