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A 30% chance of rain tomorrow: how does the public understand probabilistic weather forecasts?

Gigerenzer, Gerd, Hertwig, Ralph, van den Broek, Eva, Fasolo, Barbara and Katsikopoulos, Konstantinos V. (2005) A 30% chance of rain tomorrow: how does the public understand probabilistic weather forecasts? Risk Analysis, 25 (3). pp. 623-629. ISSN 0272-4332

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Abstract

The weather forecast says that there is a “30% chance of rain,” and we think we understand what it means. This quantitative statement is assumed to be unambiguous and to convey more information than does a qualitative statement like “It might rain tomorrow.” Because the forecast is expressed as a single-event probability, however, it does not specify the class of events it refers to. Therefore, even numerical probabilities can be interpreted by members of the public in multiple, mutually contradictory ways. To find out whether the same statement about rain probability evokes various interpretations, we randomly surveyed pedestrians in five metropolises located in countries that have had different degrees of exposure to probabilistic forecasts––Amsterdam, Athens, Berlin, Milan, and New York. They were asked what a “30% chance of rain tomorrow” means both in a multiple-choice and a free-response format. Only in New York did a majority of them supply the standard meteorological interpretation, namely, that when the weather conditions are like today, in 3 out of 10 cases there will be (at least a trace of) rain the next day. In each of the European cities, this alternative was judged as the least appropriate. The preferred interpretation in Europe was that it will rain tomorrow “30% of the time,” followed by “in 30% of the area.” To improve risk communication with the public, experts need to specify the reference class, that is, the class of events to which a single-event probability refers.

Item Type: Article
Official URL: http://www.blackwellpublishing.com/journals/RISK
Additional Information: © 2005 Wiley-Blackwell
Library of Congress subject classification: T Technology > T Technology (General)
H Social Sciences > HD Industries. Land use. Labor > HD61 Risk Management
Rights: http://www.lse.ac.uk/library/usingTheLibrary/academicSupport/OA/depositYourResearch.aspx
Date Deposited: 29 Aug 2008 09:15
URL: http://eprints.lse.ac.uk/16975/

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