We must often make a decision even though the information we have is ambiguous or uncertain. One such situation is illustrated by a patient being treated by an allergist. The patient sometimes, but not always, develops hives after eating strawberries. Moreover, the patient sometimes develops hives even when strawberries are not eaten. Another type of ambiguous situation is illustrated by the task confronted by the radiologist. The radiologist must decide whether or not a x-ray indicates the presence of lung cancer. The signals seen in the x-ray are ambiguous, some consistent with lung cancer and others inconsistent with lung cancer. Despite the obvious similarities between the tasks, they have been treated quite differently. The allergy task has often been used by researchers interested in contingency assessment; that is, how humans judge that a cue (strawberry ingestion) imperfectly signals an outcome. The cancer task has often been used by researchers interested in signal detection; that is, how humans make decisions about the presence of a signal (cancer symptoms) in a noisy background. Research concerned with contingency assessment and research concerned with signal detection have progressed independently, each with its own traditions and each motivated by different theoretical perspectives and models. I integrate these two lines of research by suggesting that contingency assessment is a form of signal detection. This psychophysical approach to the analysis of contingency judgment data provides insight into depressive realism and superstitious behaviour.