Observational studies are commonly used to evaluate the changes in outcomes associated with health care policy implementation. An important limitation in using observational studies in this context is the need to control for background changes in outcomes that occur with time (eg, secular trends affecting outcomes). The difference-in-differences approach is increasingly applied to address this problem.1
Dimick JB, Ryan AM. Methods for Evaluating Changes in Health Care Policy: The Difference-in-Differences Approach. JAMA. 2014;312(22):2401–2402. doi:10.1001/jama.2014.16153
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