Control outcomes and exposures can improve internal validity of nonrandomized studies by assessing residual bias in effect estimates. Control outcomes are those expected to have no treatment effect or the opposite effect of the primary outcome. Control exposures are treatments expected to have no effect on the primary outcome. The researchers reviewed examples of control outcomes and exposures from prior studies in Google Scholar and Medline. They found that there is inconsistent terminology for these concepts, making identification challenging. Given ongoing concern about clinical and policy inferences from nonrandomized studies, the researchers recommend adoption of these measurement tools.
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