Methods for evaluating changes in health care policy: the difference-in-differences approach.

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 In this issue of JAMA, studies by Rajaram and colleagues2 and Patel and colleagues3 used the difference-in-differences approach to evaluate the changes that occurred following the 2011 Accreditation Council for GraduateMedical Education (ACGME) duty hour reforms. The 2 studieswere conductedwith different data sources and study populations but used similar methods.