title: Test result-based sampling: an efficient design for estimating the accuracy of patient safety indicators.
abstract: Accuracy studies of Patient Safety Indicators (PSIs) are critical but limited by the large samples required due to low occurrence of most events. We tested a sampling design based on test results (verification-biased sampling [VBS]) that minimizes the number of subjects to be verified.
  
  
  
  We considered 3 real PSIs, whose rates were calculated using 3 years of discharge data from a university hospital and a hypothetical screen of very rare events. Sample size estimates, based on the expected sensitivity and precision, were compared across 4 study designs: random and VBS, with and without constraints on the size of the population to be screened.
  
  
  
  Over sensitivities ranging from 0.3 to 0.7 and PSI prevalence levels ranging from 0.02 to 0.2, the optimal VBS strategy makes it possible to reduce sample size by up to 60% in comparison with simple random sampling. For PSI prevalence levels below 1%, the minimal sample size required was still over 5000.
  
  
  
  Verification-biased sampling permits substantial savings in the required sample size for PSI validation studies. However, sample sizes still need to be very large for many of the rarer PSIs.
authors: Institute of Social and Preventive Medicine, Centre Hospitalier Universitaire Vaudois, and University of Lausanne, Lausanne, Switzerland (PT, PH, BB)
authors: Patrick
authors: P
authors: Taffé
authors: Institute of Social and Preventive Medicine, Centre Hospitalier Universitaire Vaudois, and University of Lausanne, Lausanne, Switzerland (PT, PH, BB)
authors: Patricia
authors: P
authors: Halfon
authors: Center for Health and Policy Studies, Department of Community Health Sciences, and Department of Medicine, University of Calgary, Calgary, Alberta, Canada (WAG)
authors: William A
authors: WA
authors: Ghali
authors: Institute of Social and Preventive Medicine, Centre Hospitalier Universitaire Vaudois, and University of Lausanne, Lausanne, Switzerland (PT, PH, BB)
authors: Bernard
authors: B
authors: Burnand
mesh_terms: D000328:Adult; D000465:Algorithms; D016001:Confidence Intervals; D006785:Hospitals, University / Q000592:standards; D006801:Humans; D007297:Inpatients; D019300:Medical Errors / Q000517:prevention & control*; D008875:Middle Aged; D061214:Patient Safety / Q000706:statistics & numerical data*; D019984:Quality Indicators, Health Care / Q000592:standards*; D012680:Sensitivity and Specificity