Two Techniques for Teaching the Estimation of Prior Probabilities

BACKGROUND: Studies indicate that practicing clinicians do not employ quantitative techniques in diagnosis. If evidence-based approaches to clinical care are to succeed, there is a need to develop pedagogical tools to introduce the relevant techniques in clinical medicine. DESCRIPTION: This article describes 2 heuristic techniques that can be used by clinical teachers to introduce residents and medical students to the practice of estimating prior probabilities in diagnostic reasoning. The techniques are the probability pie and the probability dollar. The techniques require learners to divide either a pie or a dollar according to the probability they assign in the differential diagnosis. The techniques also introduce related ideas such as likelihood ratios, posterior probabilities, diagnostic accuracy and intra- and interrater variability. EVALUATION: An example of a 60-year-old man presenting with chest pain is presented to illustrate the technique. CONCLUSION: This article presents a simple and clinically relevant method of introducing important concepts in quantitative reasoning. More systematic evaluation is required to assess its effectiveness.