What is the effect of area size when using local area practice style as an instrument?

Abstract

Objectives: 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.

Study design: 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.

Conclusion: 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.

Keywords: Acute myocardial infarction; Angiotensin converting enzyme inhibitors and angiotensin receptor blockers; Instrumental variables; Local area practice styles; Local average treatment effects; Survival.

Figure 1

First-Stage F-Statistics for the Effect of Local area Practice Style on ACE/ARB Use by Local Area Size (10 Patients to 200 Patients)

Figure 2

Figure 2a: Northeast United States ACE/ARB Area Treatment Ratios Based on Local Areas Defined Using 10 Patients Around 5-Digit ZIP Codes. Figure 2b: Northeast United States ACE/ARB Area Treatment Ratios Based on Local Areas Defined Using 200 Patients Around 5-Digit ZIP Codes.

Figure 3

Over-Identification F-Statistics by Local Area Size (10 Patients to 200 Patients)

Figure 4

Correlations of Local Area ACE/ARB Area Treatment Ratio and Local Area Overall Life Expectancy by Local Area Size (10 Patients to 200 Patients)

Figure 5

Instrumental Variable Parameter Estimates and Standard Errors for the Effect of ACE/ARB Prescribing Post AMI on 1-Year Survival and Standard Error by Local Area Size (10 Patients to 200 Patients)