The purpose of the study was to determine whether students had knowledge of the extension rule (the conjunction of two or more events cannot be greater than the probability of any one of those events) and understanding of conjunction by giving a test of multiplicative probabilities. Involved were 598 students enrolled in introductory psychology courses. A 10-item test of mathematical skills was given; it contained simple, additive, and multiplicative probability problems. Students were assigned randomly to either an instructional set condition or no instructional set condition. It was found that the majority of students were influenced by the representativeness heuristic. Overall, 83% of the students failed to use the extension rule to evaluate the conjunction problem. The vast majority (85%) did not answer either of the multiplicative problems correctly, thus indicating a failure to calculate objective compound probabilities. (MNS)