Calculating the number needed to treat for trials where the outcome is time to an event

The number needed to treat is the number of patients who need to be treated to prevent one additional adverse outcome. This number (with confidence interval) is a clinically useful way to report the results of controlled trials. For any trial which has reported a binary outcome, the number needed to treat can be obtained as the reciprocal of the absolute difference in proportions of patients with the outcome of interest. In studies where the outcome of interest is the time to an event, calculations can be extended to show the number needed to treat at any time after the start of treatment.