Surveying physicians to determine the minimal important difference: Implications for sample-size calculation

The minimal important difference (MID) is the smallest benefit of treatment that would result in clinicians recommending it to their patients. The MID is necessary to calculate sample size for randomized clinical trials, but its chosen value is often arbitrary. This study set out to determine the pr acticability of surveying physicians to elicit the MID for clinical trial s ample-size calculation. Using a mail survey, we elicited the MID of differe nt physician specialties (family medicine, internal medicine, vascular surg ery) for using propranolol to slow abdominal aortic aneurysm (AAA) growth a ssuming that propranolol was efficacious in this condition. We used differe nt outcome measures (growth rate or proportion of patients requiring surger y) and different methods of data presentation for the proportion of patient s requiring surgery (absolute risk reduction or number needed to treat). Th e MID varied significantly by physician specialty, experience with AAA and propranolol, and the method used to elicit the MID. Consequently, sample-si ze calculations using these various MIDs varied from 116 to 3015. Future at tempts to elicit the MID need tu consider carefully who is surveyed, how da ta are presented, and how opinions are elicited. J CLIN EPIDEMIOL 52;8:717- 723, 1999. (C) 1999 Elsevier Science Inc.