Testing the descriptive validity of possibility theory in human judgments of uncertainty

Many works in the past showed that human judgments of uncertainty do not conform very well to Probability Theory. The present paper reports four experiments that were conducted in order to evaluate if human judgments of uncertainty conform better to Possibility Theory. At first, two experiments investigate the descriptive properties of some basic possibilistic measures. Then a new measurement apparatus is used, the Ψ-scale, to compare possibilistic vs. probabilistic disjunction and conjunction. Results strongly suggest that a human judgment is qualitative in essence, closer to a possibilistic than to a probabilistic approach of uncertainly. The paper also describes a qualitative heuristic, for conjunction, that was used by expert radiologists.