title: The 1-in-X effect on the subjective assessment of medical probabilities.
abstract: Among numerical formats available to express probability, ratios are extensively used in risk communication, perhaps because of the health professional's intuitive sense of their clarity and simplicity. Moreover, health professionals, in the attempt to make the data more meaningful, tend to prefer proportions with a numerator of 1 and shifting denominators (e.g., 1 in 200) rather than equivalent rates of disease per unit of population exposed to the threat (e.g., 5 in 1000). However, in a series of 7 experiments, it is shown that individual subjective assessments of the same probability presented through proportions rather than rates vary significantly. A 1-in-X format (e.g., 1 in 200) is subjectively perceived as bigger and more alarming than an N-in-X*N format (e.g., 5 in 1000). The 1-in-X effect generalizes to different populations, probabilities, and medical conditions. Further-more, the effect is not attenuated by a communicative intervention (verbal analogy), but it disappears with an icon array visual aid.
authors.affiliation: Centre National de la Recherche Scientifique and Université de Toulouse, Toulouse, France (SP, J-FB)
authors.forename: Stefania
authors.initials: S
authors.lastname: Pighin
authors.affiliation: Department of Cognitive Science and Education, University of Trento, Trento, Italy (LS, EB)
authors.forename: Lucia
authors.initials: L
authors.lastname: Savadori
authors.affiliation: Department of Cognitive Science and Education, University of Trento, Trento, Italy (LS, EB)
authors.forename: Elisa
authors.initials: E
authors.lastname: Barilli
authors.affiliation: HSR–University Hospital of San Raffaele, Milan, Italy (LC, MF)
authors.forename: Laura
authors.initials: L
authors.lastname: Cremonesi
authors.affiliation: HSR–University Hospital of San Raffaele, Milan, Italy (LC, MF)
authors.forename: Maurizio
authors.initials: M
authors.lastname: Ferrari
authors.affiliation: Centre National de la Recherche Scientifique and Université de Toulouse, Toulouse, France (SP, J-FB)
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Bonnefon
mesh_terms: D000328:Adult; D003657:Decision Making; D005260:Female; D006801:Humans; D011247:Pregnancy; D011336:Probability; D011795:Surveys and Questionnaires