title: Relevance of outlier cases in case mix systems and evaluation of trimming methods.
abstract: OBJECTIVE
  To determine the most appropriate outlier trimming method when the main source of information for case mix classification is length of stay (LOS) because cost information is unavailable.
  
  
  METHODS
  Discharges (35,262) from two public hospitals were analysed. LOS and cost outliers were calculated using different trimming methods. The agreement between cost and LOS trimming was analysed.
  
  
  RESULTS
  The trimming method using the geometric mean with two standard deviations (GM2) showed the highest level of agreement between cost and LOS and revealed the greatest proportion of extreme costs. Nearly 5% of cases were outliers, containing 16% of total LOS. This was the best approximation to 18% of extreme cost because when GM2 was applied to LOS, 88% of outlier cost was revealed.
  
  
  CONCLUSIONS
  The methods were analysed because they are the most frequently used but the same methodology could be employed to compare other outlier determination methods. Outliers should be calculated because they ought to be valued differently from inlier cases.
authors.affiliation: Municipal Institute of Health, Health Service Research Unit, Hospital del Mar, Passeig Marítim 25-29, E-08003 Barcelona, Spain. fcots@imas.imim.es
authors.forename: Francesc
authors.initials: F
authors.lastname: Cots
authors.forename: David
authors.initials: D
authors.lastname: Elvira
authors.forename: Xavier
authors.initials: X
authors.lastname: Castells
authors.forename: Marc
authors.initials: M
authors.lastname: Sáez
mesh_terms: D003953:Diagnosis-Related Groups / Q000191:economics / Q000706:statistics & numerical data*; D017721:Hospital Costs / Q000706:statistics & numerical data; D006801:Humans; D007902:Length of Stay / Q000191:economics / Q000706:statistics & numerical data*; D017059:Models, Econometric; D016311:Outliers, DRG / Q000191:economics / Q000706:statistics & numerical data*; D010351:Patient Discharge; D012372:ROC Curve; D013030:Spain