Misinterpretations of Precipitation Probability Forecasts

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Previous studies have suggested that the general public misinterprets probability of precipitation (PoP) forecasts, leading some meteorologists to argue that probabilities should not be included in public weather forecasts. Upon closer examination, however, these studies prove to be ambiguous with regard to the nature of the misunderstanding. Is the public confused about the meaning of the probabilities or about the definition of the event to which the probabilities refer? If event misinterpretation is the source of the confusion, then elimination of the probabilities would not reduce the level of misunderstanding.

The present paper summarizes a study of 79 residents of Eugene, Oreg., who completed a questionnaire designed to investigate their understanding of and attitude toward precipitation probability forecasts. Results indicate that the event in question frequently is misunderstood, with both traditional precipitation forecasts and PoP forecasts producing similar levels of event misinterpretation. On the other hand, the probabilities themselves are well understood. Moreover, most respondents revealed a preference for the use of probabilities to express the uncertainty inherent in precipitation forecasts. Although the sample size was limited, the results of this study strongly support the inclusion of probabilities in public forecasts of precipitation occurrence. The paper concludes with a brief discussion of some implications of these results for operational weather forecasting.

1 Department of Atmospheric Sciences, Oregon State University, Corvallis, Oreg. 97331. Supported in part by the National Science Foundation under Grant ATM77-24060.

2 Decision Research, A Branch of Perceptronics, Eugene, Oreg. 97401.

3 Graduate School of Business, Indiana University, Bloomington, Ind. 47401. Supported in part by the National Science Foundation under Grant ATM77-24060.

Previous studies have suggested that the general public misinterprets probability of precipitation (PoP) forecasts, leading some meteorologists to argue that probabilities should not be included in public weather forecasts. Upon closer examination, however, these studies prove to be ambiguous with regard to the nature of the misunderstanding. Is the public confused about the meaning of the probabilities or about the definition of the event to which the probabilities refer? If event misinterpretation is the source of the confusion, then elimination of the probabilities would not reduce the level of misunderstanding.

The present paper summarizes a study of 79 residents of Eugene, Oreg., who completed a questionnaire designed to investigate their understanding of and attitude toward precipitation probability forecasts. Results indicate that the event in question frequently is misunderstood, with both traditional precipitation forecasts and PoP forecasts producing similar levels of event misinterpretation. On the other hand, the probabilities themselves are well understood. Moreover, most respondents revealed a preference for the use of probabilities to express the uncertainty inherent in precipitation forecasts. Although the sample size was limited, the results of this study strongly support the inclusion of probabilities in public forecasts of precipitation occurrence. The paper concludes with a brief discussion of some implications of these results for operational weather forecasting.

1 Department of Atmospheric Sciences, Oregon State University, Corvallis, Oreg. 97331. Supported in part by the National Science Foundation under Grant ATM77-24060.

2 Decision Research, A Branch of Perceptronics, Eugene, Oreg. 97401.

3 Graduate School of Business, Indiana University, Bloomington, Ind. 47401. Supported in part by the National Science Foundation under Grant ATM77-24060.

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