A0540
Title: Effect aliasing in observational studies
Authors: Paul Rosenbaum - University of Pennsylvania (United States)
Jose Zubizarreta - Harvard University (United States) [presenting]
Abstract: In experimental design, aliasing of effects occurs in fractional factorial experiments, where certain low-order factorial effects are indistinguishable from certain high-order interactions: low-order contrasts may be orthogonal to one another, while their higher-order interactions are aliased and not identified. In observational studies, aliasing occurs when certain combinations of covariates, e.g., time period and various eligibility criteria for treatment, perfectly predict the treatment that an individual will receive, so a covariate combination is aliased with a particular treatment. In this situation, when a contrast among several groups is used to estimate a treatment effect, collections of individuals defined by contrast weights may be balanced with respect to summaries of low-order interactions between covariates and treatments but necessarily not balanced with respect to summaries of high-order interactions between covariates and treatments. A theory of aliasing is developed in observational studies, illustrating that theory in an observational study whose aliasing is more robust than conventional difference-in-differences, and a new form of matching is developed to construct balanced confounded factorial designs from observational data.
