Subject pool sign-up procedures: A threat to external validity

The extent to which introductory psychology students' self-selection into experiments produced biased samples was assessed in two experiments. As part of the normal sign-up procedure, students chose between apparently different experiments with quite different descriptions and between individual and group sign-up slots. Upon arriving at the laboratory, all students filled out personality questionnaires. Results demonstrated that students sorted themselves into experiments as well as into individual group sign-up slots in relation to personality dimensions. The resulting samples in each experiment were thus non-representative of even the restricted introductory psychology subject population. Implications and recommendations for the use of such populations were discussed.