Background. Physicians and medical students may lack sufficient numeracy skills to make treatment decisions, interpret test results, and practice evidence-based medicine. We evaluated whether the use of a tree diagram without numerical values as an aid for numerical processing might improve students’ test results when dealing with percentages. Methods. A prospective randomized study was carried out with 102 third-year students. Participants received 3 diagnostic test problems and were asked to determine positive predictive values. The information in these tests was expressed either in (1) natural frequencies, (2) conditional probabilities, or (3) conditional probabilities with a tree diagram without numbers. Results. Ninety-eight (96.1%) complete data sets could be obtained. The group working with natural frequencies achieved significantly higher test results (n = 29, mean score: 1.1, P = 0.045) than the group working with conditional probabilities (n = 34, mean score: 0.56). The students who were given a tree diagram in addition to conditional probabilities (n = 35, mean score: 1.26) also achieved significantly better scores than the group with conditional probabilities alone (P = 0.008). The difference between the group who had received natural frequencies and the group working with conditional probabilities and the tree diagram was not significant. Conclusions. We suggest the use of a tree diagram as a visual aid when dealing with diagnostic tests expressed in conditional probabilities.
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