The 1-in-X effect on the subjective assessment of medical probabilities

Stefania Pighin, Lucia Savadori, Elisa Barilli, Laura Cremonesi, Maurizio Ferrari, Jean François Bonnefon

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)721-729
Number of pages9
JournalMedical Decision Making
Issue number5
Publication statusPublished - Sep 2011


  • numerical risk format
  • probability assessment
  • risk communication

ASJC Scopus subject areas

  • Health Policy
  • Medicine(all)


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