Epidemiology – Numbers Needed to Treat (NNT)


NNT = the number of patients that need to be treated in order for 1 extra patient to benefit

Alternatives to NNT include:

  • Numbers Needed to Screen (NNS = No. needed to be screened for 1 to benefit)
  • Numbers Needed to Harm (NNH = No. needed to be exposed to a risk factor for 1 to be harmed)


  • NNT = 1/ARR
  • Absolute Risk Reduction (ARR) is calculated by the difference between the rate of event in controls and the rate of event in cases = (a/a+c) – (b/b+d)
  • NNTs should always be reported with 95% Confidence Intervals for interpretation


  • The lower the NNT the better.
  • E.g. Drug FAB helps prevent strokes and has an NNT of 1.  By treating Bob with FAB this should prevent him having a stroke.  On the otherhand, drug BAD has an NNT of 50, so you would have to treat 50 Bobs in order to prevent one stroke.
  • If the treatment or exposure if harmful (i.e. result is a negative number) the omit the sign and measure is renamed as NNH


  • Useful to communicate benefit and harm – easy to understand (risk communication)
  • NNTs can be used either for summarising the results of trials
  • A clinically useful measure of the relative benefit of an active treatment over a control (better than RR or OR)
  • Takes into account the frequency of the outcome – thus reflects the ublic health impact of the intervention


  • Cannot be used for performing a meta-analysis. Pooled NNTs derived from meta-analyses can be seriously misleading because the baseline risk often varies appreciably between the trials
  • Do not compare NNTs for different therapies *unless* the baseline risks of the disease are similar…

For further info, check out http://www.thennt.com/the-nnt-explained/


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