[Eeglablist] Random selection of trials

Tim Curran Tim.Curran at Colorado.EDU
Thu Apr 1 10:41:31 PDT 2010


Hi Kris,

For ERP amplitudes, Luck (2005, p 231) explains that different numbers of trials per condition biases comparisons based on peak amplitude but not mean amplitude.  

Luck, S. J. (2005). An introduction to the event-related potential technique. Cambridge, MA: MIT Press.

I have randomly selected like this before, by starting with the number of trials in the lower condition and randomly selecting the same number of trials from the higher condition, on a per subject basis.  Should be easy to do in matlab, but not sure about within eeglab.

I am also not sure if similar concerns apply to EEG time/frequency analyses.

Tim



On Mar 31, 2010, at 9:04 AM, Kris Baetens wrote:

> Dear colleagues,
> 
> We employ a paradigm which inherently leads to a different number of trials in both our conditions (oddball-like). We have two conditions, one with an average of about 150 trials, the other with about 1500 (artefact-free).
> 
> - Does anybody have research to support my concern that comparing both conditions with the total number of trials may lead to artificial effects due to the different number of trials (and associated variance and "cleanliness" of the gavg's)? (I have seen such things published before.)
> - Does anybody know of an easy way to make a random selection of a predetermined number of trials out of the total number in EEGLAB or MATLAB? (Which would allow for selecting an equal number of trials in both conditions.) Obviously, we don't simply want to take the first or last 150 regular trials, since this would possibly lead to erronous conclusions. 
> 
> Thank you very much in advance,
> 
> Kris Baetens
> Ph.D. fellow of the Research Foundation - Flanders (FWO) 
> Dept. Experimental and Applied Psychology
> Faculty of Psychology and Educational Sciences
> Vrije Universiteit Brussel
> 
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