Abstract
Evidence-based policymaking is becoming the norm, but how do we acquire the evidence to inform policies? In their article in the present issue of the Journal, Basu et al. (Am J Epidemiol. 2016;183(6):531-538) used difference-in-difference-in-differences models and a synthetic control approach to examine the effects of the 1996 welfare reforms on health outcomes among single mothers. In the present commentary, we discuss the limitations of observational studies for policy evaluation. Difference-in-differences models, from the field of economics, offer a rigorous approach to cope with those limitations.