Understanding ‘Unlikely (20% Likelihood)’ or ‘20% Likelihood (Unlikely)’ Outcomes: The Robustness of the Extremity Effect

• Published in: Journal of behavioral decision making Vol. 31; no. 4; pp. 572 - 586
• Main Authors: Jenkins, Sarah C., Harris, Adam J.L., Lark, R.M.
• Format: Journal Article
• Language: English
• Published: Chichester Wiley Periodicals Inc 01-10-2018
• Subjects: Communication; extremity effect; geological hazards; Inconsistency; mixed-format; numerical probabilities; risk communication; Robustness; Uncertainty; verbal probability expressions
• ISSN: 0894-3257; 1099-0771
• DOI: 10.1002/bdm.2072

Calls to communicate uncertainty using mixed, verbal‐numerical formats (‘unlikely [0–33%]’) have stemmed from research comparing mixed with solely verbal communications. Research using the new ‘which outcome’ approach to investigate understanding of verbal probability expressions suggests, however, that mixed formats might convey disadvantages compared with purely numerical communications. When asked to indicate an outcome that is ‘unlikely’, participants have been shown to often indicate outcomes with a value exceeding the maximum value shown, equivalent to a 0% probability —an ‘extremity effect’. Recognising the potential consequences of communication recipients expecting an ‘unlikely’ event to never occur, we extend the ‘which outcome’ work across four experiments, using verbal, numerical, and verbal‐numerical communication formats, as well as a previously unconsidered numerical‐verbal format. We examine how robust the effect is in the context of consequential outcomes and over non‐normal distributions. We also investigate whether participants are aware of the inconsistency in their responses from a traditional ‘how likely’ and ‘which outcome’ task. We replicate and extend previous findings, with preference for extreme outcomes (including above maximum values) observed in both verbal and verbal‐numerical formats. Our results suggest caution in blanket usage of recently recommended verbal‐numerical formats for the communication of uncertainty. Copyright © 2018 John Wiley & Sons, Ltd.