From elementary probability theory, the probability of a conjunction P(A & B) cannot exceed the probability of either of its constituents, P(A) or P(B). This is the conjunction rule. However, it is often the case that the conjunction is more representative of its class than either of its constituents, or more available in some way, and therefore judgements of its probability are subject to one of the representativeness or availability heuristics.

In the example, A. was erroneously selected by 92% of subjects including those who were informed in matters of statistics.