Judgment and Decision Making

10

FIFTY–FIFTY = 50%?

Baruch Fischhoff and Bruine de Bruin

CARNEGIE MELLON UNIVERSITY, USA

© 1999 JOHN WILEY AND SONS, LTD.

Abstract

Several recent surveys have asked respondents to estimate the probabilities of relatively unlikely events, such as dying from breast cancer and smoking. Examination of their response distributions reveals a seemingly inappropriate ‘blip’ at 50. The two studies reported here indicate that it reflects a response artifact associated with open-ended probability scales. The blip vanishes when a response scale with explicit response options is offered. Apparently, the open-ended format leads some people to use the 50% option as ‘fifty–fifty’, an expression of having no idea as to the answer. As a result, the accuracy ...