title: Does Categorization Method Matter in Exploring Volume-Outcome Relation? A Multiple Categorization Methods Comparison in Coronary Artery Bypass Graft Surgery Surgical Site Infection.
abstract: BACKGROUND
  Volume-infection relation studies have been published for high-risk surgical procedures, although the conclusions remain controversial. Inconsistent results may be caused by inconsistent categorization methods, the definitions of service volume, and different statistical approaches. The purpose of this study was to examine whether a relation exists between provider volume and coronary artery bypass graft (CABG) surgical site infection (SSI) using different categorization methods.
  
  
  METHODS
  A population-based cross-sectional multi-level study was conducted. A total of 10,405 patients who received CABG surgery between 2006 and 2008 in Taiwan were recruited. The outcome of interest was surgical site infection for CABG surgery. The associations among several patient, surgeon, and hospital characteristics was examined. The definition of surgeons' and hospitals' service volume was the cumulative CABG service volumes in the previous year for each CABG operation and categorized by three types of approaches: Continuous, quartile, and k-means clustering.
  
  
  RESULTS
  The results of multi-level mixed effects modeling showed that hospital volume had no association with SSI. Although the relation between surgeon volume and surgical site infection was negative, it was inconsistent among the different categorization methods.
  
  
  CONCLUSIONS
  Categorization of service volume is an important issue in volume-infection study. The findings of the current study suggest that different categorization methods might influence the relation between volume and SSI. The selection of an optimal cutoff point should be taken into account for future research.
authors.affiliation: Institute of Health Policy and Management, National Taiwan University , Taipei, Taiwan .
authors.forename: Tsung-Hsien
authors.initials: TH
authors.lastname: Yu
authors.affiliation: Institute of Health Policy and Management, National Taiwan University , Taipei, Taiwan .
authors.forename: Yu-Chi
authors.initials: YC
authors.lastname: Tung
authors.affiliation: Institute of Health Policy and Management, National Taiwan University , Taipei, Taiwan .
authors.forename: Kuo-Piao
authors.initials: KP
authors.lastname: Chung
mesh_terms: D000368:Aged; D016000:Cluster Analysis; D001026:Coronary Artery Bypass / Q000009:adverse effects* / Q000706:statistics & numerical data*; D003430:Cross-Sectional Studies; D005260:Female; D006761:Hospitals / Q000706:statistics & numerical data*; D006801:Humans; D008297:Male; D008875:Middle Aged; D066231:Surgeons / Q000706:statistics & numerical data*; D013530:Surgical Wound Infection / Q000453:epidemiology* / Q000209:etiology*