Fifty-fifty=50%?

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 of people's reported beliefs depends on the response scale used, as well as on how it evokes and channels such feelings of epistemic uncertainty.