Objective: To compare clinicians' ratings of therapeutic effectiveness when different trial end points were presented as percent reductions in relative compared with absolute risk and as numbers of patients treated to avoid one adverse outcome.

Design: Survey, with random allocation of two questionnaires.

Setting: Toronto teaching hospitals.

Respondents: Convenience sample of 100 faculty and housestaff in internal medicine and family medicine.

Intervention: One questionnaire presented results for three end points of the Helsinki Heart Study as separate drug trials using only absolute differences in events; the other showed the same end points as relative differences. Both questionnaires included a fourth "trial," showing person-years of treatment needed to prevent one myocardial infarction.

Main outcome measure: The "trials" were each rated on an 11-point scale, from treatment "harmful" to "very effective."

Results: Respondents' ratings of effectiveness varied with the end point. Controlling for end point, ratings of effectiveness by the 50 participants receiving absolute event data were lower than those by 50 participants responding to relative risk reductions (P < 0.001); however, no end-point difference was more than 0.6 scale points. For a "trial" reporting that 77 persons were treated for 5 years to prevent one myocardial infarction, mean ratings were 2.3 or 1.8 scale points lower, respectively (both P < 0.001), than when the same data were shown as relative or absolute risk reductions.

Conclusions: Clinicians' views of drug therapies are affected by the common use of relative risk reductions in both trial reports and advertisements, by end-point emphasis, and, above all, by underuse of summary measures that relate treatment burden to therapeutic yields in a clinically relevant manner.