Background: The number needed to treat (NNT) has been widely recommended for communicating benefits from risk reductions. It has been claimed that NNT is easily understood and that it has intuitive meaning. There are, however, only few studies of lay people's understanding of NNT.
Objective: The objective of this study was to explore whether lay people are sensitive to the magnitude of treatment effectiveness as expressed in terms of NNT, and whether the sensitivity is influenced by variation in the type of health outcome, variation in patients' payment for the treatment or variation in the type of side effects.
Methods: In total, 2326 non-institutionalized Danes aged 18-91 years were interviewed face to face and asked whether they would consent to a treatment against a somewhat increased risk of death or heart attack. The respondents were allocated to different levels of effectiveness of treatment expressed as NNT of 10, 100, 200 or 400, different costs and different descriptions of adverse effects.
Results: The odds for consenting to therapy were little influenced by the magnitude of NNT, but were greater among married respondents and among those who had side effects presented in terms of headache and constipation.
Conclusion: In this study, the respondents' choice of treatment was largely insensitive to the magnitude of NNT independently of variations in type of health outcome and costs. NNT may not be easily understood by lay people.