Improbable outcomes: Infrequent or extraordinary?

Research on verbal probabilities has shown that unlikely or improbable events are believed to correspond to numerical probability values between 10% and 30%. However, building on a pragmatic approach of verbal probabilities and a new methodology, the present paper shows that unlikely outcomes are most often associated with outcomes that have a 0% frequency of occurrence. Five studies provide evidence that when people complete or evaluate statements describing “improbable” outcomes, based on outcome distributions or expected ranges, they favor extraordinary outcomes that have not occurred in the original sample. For quantitative outcomes that can be ordered on a unipolar dimension, an improbable outcome is typically perceived as having a higher outcome value than those observed. Thus when battery life for a sample of laptop batteries is shown to range from 2.5 to 4.5 h, 5 or 6 h are considered better examples of “improbable” duration times than those that actually occur in 10% of the cases. Similarly, an improbable exam grade is one that has not yet been observed, rather than one that has been obtained by a small percentage of students. And when climate experts claim that a 100 cm increase in sea level by the year 2100 is “improbable”, participants believe that the same experts’ maximum estimates will be much lower. We conclude that judgments of what is improbable suggest outcomes beyond the expected range, rather than simply low frequency outcomes. These results are compatible with a causal (propensity) interpretation rather than a statistical (frequency) interpretation of probabilities.