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JAMA Guide to Statistics and Methods |

Methods for Evaluating Changes in Health Care Policy The Difference-in-Differences Approach

Justin B. Dimick, MD, MPH1,2,3,4; Andrew M. Ryan, PhD1,2,4
[+] Author Affiliations
1The Center for Healthcare Outcomes and Policy, University of Michigan, Ann Arbor
2Institute for Healthcare Policy and Innovation, University of Michigan, Ann Arbor
3Department of Surgery, School of Medicine, University of Michigan, Ann Arbor
4Department of Health Policy & Management, School of Public Health, University of Michigan, Ann Arbor
JAMA. 2014;312(22):2401-2402. doi:10.1001/jama.2014.16153.
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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 Graduate Medical Education (ACGME) duty hour reforms. The 2 studies were conducted with different data sources and study populations but used similar methods.

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Conceptual Illustration of a Difference-in-Differences Analysis for 2 Scenarios
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