title: Effect of varying the case mix on the standardized mortality ratio and W statistic: A simulation study.
abstract: OBJECTIVE
  To evaluate the validity of using the standardized mortality ratio (SMR) and the W statistic as risk-adjusted measures of hospital mortality to judge ICU performance.
  
  
  METHODS
  APACHE (acute physiology and chronic health evaluation) II data were collected prospectively from the surgical ICU (SICU) at a single institution using all adult admissions (n = 6806) over an 8-year period (excluding cardiac surgical patients, burn patients, and patients under 16 years of age). Using a computer simulation technique, virtual ICUs (VICUs) with mortality rates between 5% and 16% were constructed. After first dividing the original data set into deciles of risk, each VICU was constructed by randomly resampling between 10 and 680 patients from each decile. The SMR, W statistic, and Z statistic were calculated for 10,000 different case mixes.
  
  
  METHODS
  The SICU at a 450-bed teaching hospital.
  
  
  METHODS
  A group of 6,806 adult patient admissions, excluding cardiac surgical patients and burn patients.
  
  
  RESULTS
  VICUs were created from a data set of actual patients treated at one institution in order to test the hypothesis that the SMR and W statistic would remain invariant when applied to subsets of patients from a single institution. Instead, the SMR and W statistic were found to be very sensitive to changes in case mix. The SMR and W statistic were linear functions of the simulated ICU mortality rate.
  
  
  CONCLUSIONS
  This simulation demonstrates that the SMR and the W statistic based on APACHE II cannot be used to compare outcomes of ICUs. We have proposed a revision of the SMR that eliminates the effect of case mix and allows for more accurate comparisons of ICU performance.
authors.affiliation: Department of Anesthesiology, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642, USA.
authors.forename: L G
authors.initials: LG
authors.lastname: Glance
authors.forename: T
authors.initials: T
authors.lastname: Osler
authors.forename: T
authors.initials: T
authors.lastname: Shinozaki
mesh_terms: D018806:APACHE; D000328:Adult; D016001:Confidence Intervals; D003953:Diagnosis-Related Groups / Q000706:statistics & numerical data*; D017052:Hospital Mortality; D006784:Hospitals, Teaching / Q000706:statistics & numerical data; D006801:Humans; D007362:Intensive Care Units / Q000706:statistics & numerical data*; D015233:Models, Statistical; D011446:Prospective Studies; D012372:ROC Curve; D015203:Reproducibility of Results; D015996:Survival Rate