title: Matching on provider is risky.
abstract: OBJECTIVE
  To illustrate that matching on provider may exacerbate, not remove, bias.
  
  
  METHODS
  The degree of confounding bias depends in part on the proportions of treatment variation that can be ascribed to confounders and to instruments, respectively. This commentary raises the specific example of bias by matching on hospital induced in a study of coronary artery bypass graft surgery patients and illustrates the effect of matching on provider in a constructed example.
  
  
  RESULTS
  Matching on provider removes a "benign" source of treatment variability, leaving unmeasured confounders as potentially the most important determinants of treatment.
  
  
  CONCLUSIONS
  Researchers need to articulate the presumed source of pseudorandom variation in observational studies and need to take care not to reduce their effect through unnecessary control.
authors.affiliation: World Health Information Science Consultants, Riverside Center, Suite 2-400, 275 Grove Street, Newton, MA 02466, USA. Alec.Walker@WHISCON.com
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Walker
mesh_terms: D000614:Aminocaproates / Q000008:administration & dosage / Q000627:therapeutic use; D007611:Aprotinin / Q000008:administration & dosage / Q000627:therapeutic use; D015982:Bias; D057186:Comparative Effectiveness Research / Q000379:methods*; D015986:Confounding Factors, Epidemiologic; D001026:Coronary Artery Bypass / Q000379:methods / Q000401:mortality; D003627:Data Interpretation, Statistical; D006490:Hemostatics / Q000008:administration & dosage / Q000627:therapeutic use; D006761:Hospitals; D006801:Humans; D016555:Matched-Pair Analysis; D018579:Patient Selection; D016032:Randomized Controlled Trials as Topic; D015203:Reproducibility of Results; D012107:Research Design
chemical_list: D000614:Aminocaproates; D006490:Hemostatics; D007611:Aprotinin
keywords: Amplification bias; Comparative effectiveness research; Instrumental variables; Matching; Pseudorandomization; Unmeasured confounders; Z-bias