Disjunction errors in qualitative likelihood judgment

Abstract

Probability theory requires that the disjunction of two events be judged at least as likely as either of those events. Two studies are described which imply that violations of this requirement are very common. The data also show that subjects do not commit disjunction errors because they misinterpret the disjunctive statement "A or B" to mean "A or B, but not both." Instead, the pattern of errors is consistent with a judgment process describable as averaging. These results provide support for a "signed combination model of qualitative likelihood judgment." This model generalizes a previous one which applied to judgments concerning conjunctive events (J. F. Yates and B. W. Carlson, 1986,Organizational Behavior and Human Decision Processes, 37, 230-253).