[Eeglablist] ERP grand average does not reflect actual latency

[Akshay Maggu]

Dear Alvin,
It's always a good idea to draw scatterplots (and bars with SD or SE) of P3
latency and amplitude data to see if the variability is being caused by
some outliers. Grandaverage not showing difference is not very unusal.
After all grandaverage is average of average in which standard deviation
has gotten rid-off (or at least not depicted in the graph) at several
stages. You might also want to draw standard error-of-the-mean alongside
your grandaverage to see how it looks.

Hope this helps,
Akshay

On Tue, Mar 28, 2017 at 9:30 AM, Shih-Chun Kao <shihchunkao at gmail.com>
wrote:

> Dear list,
>
> If I found a statistically significant P3 latency effect (~50ms) between
> two conditions in a within-subject design study but my grand average plot
> does not show such difference, what would be the cause for inconsistent
> results and what does such inconsistency suggest? I understand grand
> average is not the best illustration but I thought 50ms is a pretty big
> difference. Thank you!
>
> Alvin
>
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-- 
Akshay Raj Maggu,
Laboratory for Language, Learning and the Brain,
Department of Linguistics and Modern Languages,
The Chinese University of Hong Kong,
Hong Kong.
Webpage: http://www.cuhk.edu.hk/lin/new/people/students/maggu/
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