# [Eeglablist] ICA & percent variance accounted for

Joëlle Albrecht joelle.albrecht at hotmail.com
Fri Dec 12 04:00:03 PST 2014

Dear EEGLAB-ers

I’m trying to find out, how much variance of the original
data the components (computed by ICA) explain, but I’m not sure how to do this.
(My goal is something like: Component 1 explains 60% of the variance in the
original data. etc.)

1) I used the following
function:

[pc,eigvec,sv] = runpca(EEG.data);

[proj pvaf] = compvar(EEG.data,
pc, eigvec, components);

What exactly does the „percent variance accounted for (pvaf)“
mean? For example if I got a pvaf of about 60, does this mean that this
component explains 60% of the original data (or is it more complicated)? These
pvaf’s sum up to about 80, does this mean that 20% of the original data cannot
be explained by all the components?

2) I used another function, too (after completing the ICA):

[pvaf,pvafs,vars]=eeg_pvaf(EEG,component);

Again: What exactly do the pvaf’s mean? Plus, these pvaf‘s
are very different from the pvaf’s I obtained with the function above. What is
the difference? These pvaf’s add to over 400 (whereas the ones from above sum
up to only 80 which seems to be more reasonable to me), does this mean that the
independent components overlap? The pvaf’s resulting from this function (but
not the ones from the one above)  are not
monotonously decreasing – how are they sorted?

Thank you very much for the support! Joëlle
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