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Education And Debate

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

BMJ 1999; 319 doi: (Published 04 December 1999) Cite this as: BMJ 1999;319:1492
  1. Douglas G Altman, professor of statistics in medicine (,
  2. Per Kragh Andersen, professor of biostatisticsb
  1. a Imperial Cancer Research Group Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Headington, Oxford OX3 7LF
  2. b Department of Biostatistics, University of Copenhagen, DK-2200 Copenhagen N, Denmark
  1. Correspondence to: Correspondence to: D G Altman
  • Accepted 5 July 1999

The number of patients who need to be treated to prevent one additional event (number needed to treat; NNT) has become a widely used measure of treatment benefit derived from the results of randomised controlled trials with a binary outcome 1 2 We show how to obtain a number needed to treat for studies where the primary outcome is the time to an event. We consider primarily the situation where there is no access to raw data, for example, when reviewing a published study, and also how to proceed when given the raw data.

Summary points

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

Time to event data

As noted previously, for studies with binary outcome the number needed to treat will vary according to the length of follow up.3 For studies of survival this relation with time is more explicit. There is no single number needed to treat; rather it can be calculated at any time point after the start of treatment. Often there are one or two time points of particular clinical interest.

A time specific number needed to treat represents the number of patients who need to be given the treatment in question for one additional patient to survive to that time point—that is, to benefit from …

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