Grounds, M. Joslyn, S. & LeClerc, J. (2018) Expressing Flood Likelihood: Return Period versus Probability, Weather Climate and Society 10 (1)


The likelihood of floods and other potentially destructive natural phenomena is often expressed as a return period or recurrence interval, such as a 100-yr flood. However, the expression might give users the impression that the event will occur exactly once within the described period, obscuring the intended probabilistic meaning. If so, users may think a flood is less likely when one has just occurred or more likely when it has not, leading to a “flood is due” effect. This hypothesis was tested experimentally in two studies reported here. Participants were given either a return period or a probability expression and asked to rate flood likelihood and concern. Flood recency was also manipulated. The results from both studies support a flood is due effect when the return period expression is used. In the return period condition alone, participants rated floods as more likely and expressed greater concern when no flood had occurred recently. When no likelihood information was conveyed in the control condition, the opposite effect was observed. Participants rated flood likelihood as higher and expressed greater concern when a flood had occurred recently. Participants using the percent chance expression were least affected by flood recency. This adds to the growing body of research suggesting that nonexperts can benefit from probabilistic weather forecasts.