Wighted Average for ERPs?

Discussions
1

Hello,

I was wondering if Brainstom computes a weighted average, when aveargeing a single condition across all the subjects. That is to say, if the average is computed from the normalized maps of each the subjects(in order to avoid the dominancy of one subject’s data in the average topography).

Thank you very much in advance.

Sincerely
Parvaneh

2

Hi Parvaneh,

The default behavior of the averaging function in Brainstorm is a weighted average.

All the data files contain a .nAvg field, it represents the number of files from which this file was averaged.
To check its value, right-click on the average for one subject > File > View file contents.

Let’s consider the subject averages of condition A for subjects “Subj01” and “Subj02”, calculated respectively from nA01 and nA02 trials.
If you average the two files across the two subjects, it creates a file “Avg: A” in the folder “Inter-subject”, and the field .nAvg for this file = nA01 + nA02.
The contents of this file (field .F) is calculated as a weighted average: Inter-subject.F = (nA01Subj01_A.F + nA02Subj02_A.F) ./ (nA01+nA02)

Cheers,
Francois

3

Dear François,

Thank you very much for your prompt reply.

Sincerely
Parvaneh