Background: Previous studies suggest that lay people have difficulties with evaluating effect size in terms of number needed to treat (NNT), but they are sensitive to effect size in terms of survival gains.
Aim: To explore whether GPs and internists are sensitive to NNT and survival gains when considering a lipid-lowering drug therapy.
Design and setting: Cross-sectional survey of primary prevention of cardiovascular disease with random allocation to different scenarios.
Method: GPs (n = 450) and internists (n = 450) were posted a vignette presenting a high-risk patient and a novel drug, 'neostatin'. The benefit was described in terms of NNT or mean gain in disease-free survival. Each physician was randomly allocated to one version of the vignette. Outcome measures were evaluation of 'neostatin' on a Likert scale (0: very poor choice, 10: very good choice) and the proportion recommending 'neostatin'.
Results: A total of 477 responses (53%) were received. Among responders to NNT scenarios, 26%, 31%, and 43% recommended 'neostatin' for NNT values of 34, 17, and 9 respectively. With equivalent disease-free survival gains of 9, 17, and 32 months, 40%, 49%, and 52% respectively recommended the drug. On the rating scale, mean values were 4.7, 5.0, and 5.5 across the respective NNT scenarios and 5.2, 6.2, and 6.1 across the scenarios presenting survival gains. Differences in trends between the two formats were not statistically significant. In total, 33% recommended 'neostatin' when presented with NNT values, compared to 47% when presented with survival gain (χ(2) = 9.2, P= 0.002).
Conclusion: Physicians presented with survival gains were more likely to recommend the therapy than those presented with NNT. Sensitivity to effect size was similar for both effect formats.