Set size, assertion form, thematic content and sampling in the selection task

Participants attempted to solve a modified version of Wason's (1966) selection task. Variables were: (1) sizes of the sets referenced by a specified assertion, (2) form of the assertion (conditional or categorical), (3) thematic content of the assertion (letters and numbers or makes and owners of taxicabs), and (4) the need for sampling or not. In Experiment 1, participants were given enough information to determine the truth or falsity of the specified assertion with certainty; in Experiment 2, they had to rely on sampling and could not determine the assertion's truth or falsity with certainty. Performance was better in Experiment 1 than in Experiment 2, but in both cases much better than what is typically obtained with the conventional selection task. The results support the hypothesis that performance of the selection task is sensitive to the sizes of the sets involved, add credence to the conclusion that framing the task in a thematically meaningful way can facilitate performance, and demonstrate that facilitation does not require deontic reasoning or a well-known convention being involved. The relationship between expressed confidence and level of performance is consistent with other studies showing the former to be a good predictor of the latter.