Intended for healthcare professionals

Observations Statistics in Medicine

Risky business: doctors’ understanding of statistics

BMJ 2014; 349 doi: (Published 17 September 2014) Cite this as: BMJ 2014;349:g5619
  1. Christopher Martyn, freelance writer
  1. christopher.martyn{at}

As medicine becomes increasingly preventive, doctors need a good grasp of risk

Nearly 40 years ago the New England Journal of Medicine published a short survey of doctors’ understanding of the results of diagnostic tests.1 The participants, all doctors or medical students at Harvard teaching hospitals, were asked, “If a test to detect a disease whose prevalence is 1/1000 has a false positive rate of 5%, what is the chance that a person found to have a positive result actually has the disease, assuming that you know nothing else about the person’s symptoms or signs?” This wasn’t a very difficult question, which made the results all the more shocking. Fewer than a fifth of participants gave the correct answer, and most thought that the hypothetical patient had a 95% chance of having the disease.

Of course, this was a long time ago, and medical curriculums now contain much more in the way of statistics and probabilistic reasoning. You might expect that if the exercise were repeated today almost everyone would give the right answer. But you’d …

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