How else do you expect to select volunteers? Usually, they're undergraduate students who are compensated with credit towards courses. I participated in a few ex

How else do you expect to select volunteers? Usually, they're undergraduate students who are compensated with credit towards courses. I participated in a few experiments myself as an undergrad. I presume that they administered a questionnaire where they asked, among other things, whether the volunteers have done this sort of thing before. From that you can divide people into an "experienced" and an "inexperienced" group, and compare their results.