The judgment of contingency and the nature of the response alternatives.

Previous investigations have shown that subjects make systematic errors in judging the contingency between responses and outcomes: judged contingency increases with the frequency of positive outcomes in the absence of actual contingency. In the present experiment judgments were obtained when both response alternatives were active choices (as in previous experiments), and when one alternative was to make no response. In the latter case, judgments were more accurate and were less influenced by the frequency of positive outcomes. This result was expected on the hypothesis that when both alternatives are active, subjects tend to assume that in the absence of any response no positive outcomes would occur. This faulty assumption, which is hypothesized to be one source of distortion in the judgment of contingency, is ruled out when the making of no response is an explicit alternative within the task. In what would appear to be a very simple judgment task, Jenkins & Ward (1965) found a surprising lack of correspondenc e between intuitive judgments and actual contingency. They made use of a two response—two outcome task. On each of a series of trials the subject chose to make one of two alternative responses and then observed the occurrence of one of two outcomes, a 'score' light or a 'no-score' light