Probability estimation in a chance task with changing probabilities

Subjects made sequential estimates of the probability of occurrence of one event in a chance-controlled binary-event sampling situation. Subjects whose samples were drawn from populations in which the probability of occurrence of the event in question increased tended to overestimate probabilities, while subjects whose samples were drawn from stable probability populations tended to underestimate probabilities. A second phase in which probabilities were stable for all subjects resulted in overestimation by subjects who had been in the changing probability conditions and underestimation by subjects who had been in the stable condition, although the different probability levels reached in the different conditions may have confounded these results to some extent. The results of this study show that overconfidence can be obtained in a chance controlled task in the presence of increasing probabilities, and indicate that the overconfidence of success in skill situations reported in previous studies may be due in part to the rising probabilities typically associated with skill tasks. Some suggestions for future investigations of subjective probabilities in the presence of changing objective probabilities are presented.