Common problems related to the use of number needed to treat

  • Andreas Stang
    Corresponding author. Institute of Clinical Epidemiology, Medical Faculty, Martin-Luther-University of Halle-Wittenberg, Magdeburger Strasse 8, 06097 Halle, Germany. Tel.: +49-345-557-3596; fax: +49-345-557-3565.
    Institute of Clinical Epidemiology, Medical Faculty, Martin-Luther-University of Halle-Wittenberg, Magdeburger Strasse 8, 06097 Halle, Germany
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  • Charles Poole
    Department of Epidemiology, University of North Carolina, Chapel Hill, NC, USA
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  • Ralf Bender
    Institute for Quality and Efficiency in Health Care, Department of Medical Biometry, Cologne, Germany

    Faculty of Medicine, University of Cologne, Cologne, Germany
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Published:November 02, 2009DOI:



      To illustrate basic issues that have to be taken into account when study results are presented by means of the number needed to treat (NNT).

      Study Design and Setting

      This article presents an overview of common problems related to the NNT with corresponding explanations.


      Without stating the direction of the effect, the alternative treatment, the treatment period, and the follow-up period, information in terms of NNTs is uninterpretable. The naive use of person-time data for the calculation of NNTs is frequently inappropriate. Rounding NNTs to the next upward integer may obscure differences among therapies.


      The basic information about which treatments are compared, the treatment period, the follow-up period, and the direction of the effect should be given when study results are presented in terms of NNTs. Adequate methods should be used for point and interval estimation of NNTs. Unnecessary rounding of NNTs should be avoided. In more complicated situations of confounding or varying follow-up times, the use of more sophisticated methods is required with increasing potential for misinterpretation.


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