Lead article by Olden and Møen
The article "The triple difference estimator" has been published in the September issue of the Econometrics Journal, as the editor's choice of lead article. Editor’s choice-articles are published as open access.
Econometrics Journal is on level 3 in the ABS Academic Journal Guide.
Olden, Andreas, and Jarle Møen: The triple difference estimator, Econometrics Journal, 2022, 25(3), 531-553, Online 09.03.2022 - The Editors’ Choice of lead article.
Triple difference has become a widely used estimator in empirical work. A close reading of articles in top economics journals reveals that the use of the estimator to a large extent rests on intuition. The identifying assumptions are neither formally derived nor generally agreed on.
We give a complete presentation of the triple difference estimator, and show that even though the estimator can be computed as the difference between two difference-in-differences estimators, it does not require two parallel trend assumptions to have a causal interpretation. The reason is that the difference between two biased difference-in-differences estimators will be unbiased as long as the bias is the same in both estimators. This requires only one parallel trend assumption to hold.