[Eeglablist] Random selection of trials

[Kris Baetens]

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