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Education And Debate

The number needed to treat: a clinically useful nomogram in its proper context

BMJ 1996; 312 doi: (Published 17 February 1996) Cite this as: BMJ 1996;312:426

This article has a correction. Please see:

  1. Gilles Chatellier, senior registrara,
  2. Eric Zapletal, computer scientista,
  3. David Lemaitre, computer scientista,
  4. Joel Menard, professor of internal medicineb,
  5. Patrice Degoulet, professor of medical informaticsa
  1. a Medical Informatics Department, Broussais Hospital, 75674 Paris CEDEX 14, France
  2. b Clinical Investigation Centre
  1. Correspondence to: Dr Chatellier.
  • Accepted 23 November 1995

The number needed to treat is a meaningful way of expressing the benefit of an active treatment over a control. It can be used either for summarising the results of a therapeutic trial or for medical decision making about an individual patient, but its use at the bedside has been impeded by the need for time consuming calculations. A nomogram has therefore been devised that will greatly simplify the calculations. Since calculations are now easy, the number needed to treat can be used to assess the value of several interventions, although it does have its limitations. In particular it should not be used when it is not known whether the relative risk reduction associated with an intervention is constant for all levels of risk, or for periods of time longer than that studied in the original trials.

In most medical disciplines the gold standard for evaluating the benefit of an active treatment is the randomised controlled trial. Many obstacles exist, however, to the correct use of the results of clinical trials. Inadequate dissemination of results may, for example, explain the differences between doctors in awareness of key advances in myocardial infarction.1 Another influence on clinicians' views of the effectiveness of treatments may be the way in which the results of therapeutic trials are presented.2 3 4

An informative way of presenting results is the number needed to treat described by Laupacis et al.5 As recently underlined by Cook and Sackett, this very simple index is attractive since the meaning of a sentence such as “20 patients need to be treated to avoid 1 death over a five year period” is easily understood by both doctors and patients.6 Nevertheless, the authors underline that the calculations needed—that is, the multiplying of two numbers followed by the taking of …

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