A Tutorial on Applying the Difference-in-Differences Method to Health Data

Abstract

Purpose of Review Difference-indifferences analyses are a useful tool for estimating group-level decisions, such as policy changes, training programs, or other non-randomized interventions, on outcomes which occur within the intervention group. However, there is little practical advice on how to apply difference-indifferences to epidemiologic and health data. Here, we provide a tutorial on applying the difference-indifferences method to health services data, targeted at epidemiologists and other biomedical researchers. Recent Findings As epidemiologists increasingly engage in policy discussions, familiarity with difference-indifferences will be increasingly important. However, much of the literature on difference-indifferences is limited to econometrics examples where the types of data and questions encountered may differ from health research. There remain limited resources for epi-demiologists and other medical researchers to learn how to implement difference-indifferences analyses without first having to familiarize themselves with econometric terminology and concepts. Summary This tutorial contains synthetic data, code, and worksheets for class instruction. We provide a step-by-step description of the difference-indifferences analysis including sensitivity checks, modeling decisions, and interpretation. In addition, we supply novel guidance on modeling difference-indifferences outcomes for count or score outcomes.