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Med Decis Making. 2016 Aug;36(6):686-91. doi: 10.1177/0272989X16640785. Epub 2016 Mar 31.

Natural Frequencies Do Not Foster Public Understanding of Medical Test Results.

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Center for Experimental Research in Management and Economics, DCP, University IUAV of Venice, Venice, Italy (SP, VG)
Laboratory of Cognitive Psychology, CNRS and Aix-Marseille University, Marseille, France (MG)
Department of Economics and Management, University of Trento, Trento, Italy (LS)


Major organizations recommend presenting medical test results in terms of natural frequencies, rather than single-event probabilities. The evidence, however, is that natural frequency presentations benefit at most one-fifth of samples of health service users and patients. Only one study reported a substantial benefit of these presentations. Here, we replicate that study, testing online survey respondents. Study 1 attributed the previously reported benefit of natural frequencies to a scoring artifact. Study 2 showed that natural frequencies may elicit evaluations that conflict with the normatively correct one, potentially hindering informed decision making. Ironically, these evaluations occurred less often when respondents reasoned about single-event probabilities. These results suggest caution in promoting natural frequencies as the best way to communicate medical test data to health service users and patients.


diagnostic reasoning, numeracy; natural frequencies; single-event probability; test result understanding

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