The results of three psychological studies suggest that many people did not understand probability of precipitation (PoP) despite the fact that participants were college undergraduates in the Pacific Northwest, an area with frequent precipitation forecasts. The misunderstanding concerned the class of events to which the probability refers.

The combined results reported here suggest that some participants thought that the percentage, indicating chance of precipitation, referred instead to the proportion of area or time that precipitation would be observed. More participants chose to take an umbrella or wear a hooded jacket who mistakenly interpreted the forecast as indicating precipitation for more than half of the area or time as compared to those who do not hold this misunderstanding. In addition, performance only improved significantly when PoP was accompanied by a phrase expressing the probability of no precipitation. This suggests a deep-seated misunderstanding that converts the probabilistic forecasts into a deterministic forecast for precipitation with additional information about percent time or area and affects decision making. We conclude that it is important to convey the idea that PoP is a forecast in which it is possible that no precipitation will be observed at all and make suggestions about extending uncertainty forecasts to other parameters.

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Footnotes

University of Washington, Seattle, Washington