Journal
AMERICAN JOURNAL OF EPIDEMIOLOGY
Volume 174, Issue 11, Pages 1223-1227Publisher
OXFORD UNIV PRESS INC
DOI: 10.1093/aje/kwr352
Keywords
bias (epidemiology); confounding factors (epidemiology); epidemiologic methods; instrumental variable; precision; simulation; variable selection
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Funding
- Div Of Information & Intelligent Systems
- Direct For Computer & Info Scie & Enginr [0914211] Funding Source: National Science Foundation
- NLM NIH HHS [R01 LM009961, 1R01 LM009961-01] Funding Source: Medline
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In choosing covariates for adjustment or inclusion in propensity score analysis, researchers must weigh the benefit of reducing confounding bias carried by those covariates against the risk of amplifying residual bias carried by unmeasured confounders. The latter is characteristic of covariates that act like instrumental variables-that is, variables that are more strongly associated with the exposure than with the outcome. In this issue of the Journal (Am J Epidemiol. 2011; 174(11): 1213-1222), Myers et al. compare the bias amplification of a near-instrumental variable with its bias-reducing potential and suggest that, in practice, the latter outweighs the former. The author of this commentary sheds broader light on this comparison by considering the cumulative effects of conditioning on multiple covariates and showing that bias amplification may build up at a faster rate than bias reduction. The author further derives a partial order on sets of covariates which reveals preference for conditioning on outcome-related, rather than exposure-related, confounders.
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