Design of Observational Studies pp 113-145
Opportunities, Devices, and Instruments
What features of the design of an observational study affect its ability to distinguish a treatment effect from bias due to an unmeasured covariate uij? This topic, which is the focus of Part III of the book, is sketched in informal terms in the current chapter. An opportunity is an unusual setting in which there is less confounding with unobserved covariates than occurs in common settings. One opportunity may be the base on which one or more natural experiments are built. A device is information collected in an effort to disambiguate an association that might otherwise be thought to reflect either an effect or a bias. Typical devices include: multiple control groups, outcomes thought to be unaffected by the treatment, coherence among several outcomes, and varied doses of treatment. An instrument is a relatively haphazard nudge towards acceptance of treatment where the nudge itself can affect the outcome only if it prompts acceptance of the treatment. Although competing theories structure design, opportunities, devices, and instruments are ingredients from which designs are built.
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