NATIONAL BUREAU OF ECONOMIC RESEARCH
NATIONAL BUREAU OF ECONOMIC RESEARCH
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Ariella Kahn-Lang

Harvard University
177 Winchester St
Brookline, MA 02446

E-Mail: EmailAddress: hidden: you can email any NBER-related person as first underscore last at nber dot org
Institutional Affiliation: Harvard University

NBER Working Papers and Publications

July 2018The Promise and Pitfalls of Differences-in-Differences: Reflections on ‘16 and Pregnant’ and Other Applications
with Kevin Lang: w24857
We use the exchange between Kearney/Levine and Jaeger/Joyce/Kaestner on “16 and Pregnant” to reexamine the use of DiD as a response to the failure of nature to properly design an experiment for us. We argue that 1) any DiD paper should address why the original levels of the experimental and control groups differed, and why this would not impact trends, 2) the parallel trends argument requires a justification of the chosen functional form and that the use of the interaction coefficients in probit and logit may be justified in some cases, and 3) parallel trends in the period prior to treatment is suggestive of counterfactual parallel trends, but parallel pre-trends is neither necessary nor sufficient for the parallel counterfactual trends condition to hold. Importantly, the purely statistica...
 
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