Background: The number needed to treat (NNT) has been promoted as the preferred effect measure when patients and physicians share decision making. Our aim was to explore the impact of the NNT on laypeople's decisions about preventive drug therapies.
Methods: Two thousand subjects were selected for the survey; 1201 (60%) responded for a representative sample of the Norwegian population. Respondents were allocated to scenarios with random combinations of a disease to be prevented, drug treatment costs, and effect size in terms of NNT. They were interviewed about their hypothetical consent to the therapy, then randomized to different interpretations of NNT and asked to reconsider their initial responses.
Results: The proportions consenting varied from 76% when the NNT was 50 to 67% when the NNT was 1600 (P for trend = .06). When faced with the prospect of avoiding lethal disease, stroke, myocardial infarction, or hip fracture, the proportions consenting were 84%, 76%, 68%, and 53%, respectively (P<.01). Across different treatment costs ($37, $68, $162, and $589) the proportions consenting varied from 78% to 61% (P for trend <.01). Twenty-four percent of the respondents changed their decision when informed about how to interpret the NNT, and 93% of those switched from positive to negative decisions, regardless of the magnitude of NNT.
Conclusions: Respondents' decisions were influenced by the type of disease to be prevented and the cost of the intervention, but not by the effect size in terms of NNT. This suggests that NNT is difficult to understand and that other effect formats should be considered for shared decision making.