title: What is the effect of area size when using local area practice style as an instrument?
abstract: OBJECTIVE
  Discuss the tradeoffs inherent in choosing a local area size when using a measure of local area practice style as an instrument in instrumental variable estimation when assessing treatment effectiveness.
  
  
  METHODS
  Assess the effectiveness of angiotensin converting-enzyme inhibitors and angiotensin receptor blockers on survival after acute myocardial infarction for Medicare beneficiaries using practice style instruments based on different-sized local areas around patients. We contrasted treatment effect estimates using different local area sizes in terms of the strength of the relationship between local area practice styles and individual patient treatment choices; and indirect assessments of the assumption violations.
  
  
  RESULTS
  Using smaller local areas to measure practice styles exploits more treatment variation and results in smaller standard errors. However, if treatment effects are heterogeneous, the use of smaller local areas may increase the risk that local practice style measures are dominated by differences in average treatment effectiveness across areas and bias results toward greater effectiveness.
  
  
  CONCLUSIONS
  Local area practice style measures can be useful instruments in instrumental variable analysis, but the use of smaller local area sizes to generate greater treatment variation may result in treatment effect estimates that are biased toward higher effectiveness. Assessment of whether ecological bias can be mitigated by changing local area size requires the use of outside data sources.
authors.affiliation: University of Iowa, College of Pharmacy and College of Public Health, S-515 Pharmacy Bldg., 115 S. Grand Ave, Iowa City, IA 52242, USA. john-brooks@uiowa.edu
authors.forename: John M
authors.initials: JM
authors.lastname: Brooks
authors.forename: Yuexin
authors.initials: Y
authors.lastname: Tang
authors.forename: Cole G
authors.initials: CG
authors.lastname: Chapman
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Cook
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Chrischilles
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D057911:Angiotensin Receptor Antagonists / Q000627:therapeutic use*; D000806:Angiotensin-Converting Enzyme Inhibitors / Q000627:therapeutic use*; D001334:Automobile Driving; D015982:Bias; D015986:Confounding Factors, Epidemiologic; D003627:Data Interpretation, Statistical; D005194:Family Practice / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D016018:Least-Squares Analysis; D008297:Male; D006278:Medicare; D009203:Myocardial Infarction / Q000188:drug therapy / Q000401:mortality*; D017063:Outcome Assessment, Health Care / Q000706:statistics & numerical data*; D010818:Practice Patterns, Physicians' / Q000706:statistics & numerical data*; D012111:Residence Characteristics / Q000706:statistics & numerical data*; D013997:Time Factors; D014481:United States / Q000453:epidemiology
chemical_list: D057911:Angiotensin Receptor Antagonists; D000806:Angiotensin-Converting Enzyme Inhibitors
keywords: Acute myocardial infarction; Angiotensin converting enzyme inhibitors and angiotensin receptor blockers; Instrumental variables; Local area practice styles; Local average treatment effects; Survival