Calculation of Number Needed to Treat

To the Editor: The number of patients who would need to be treated to prevent a given adverse outcome in one patient, called the number needed to treat, is often used in randomized trials and observational studies to provide a simple measure of the effect of a treatment. The computation of the number needed to treat can, however, be inaccurate and its interpretation misleading in trials with varying follow-up times. In this case, the cumulative incidence of an outcome cannot simply be calculated as a proportion of subjects but must instead be estimated over time by means of the Kaplan–Meier . . .