Population Interference in Panel Experiments

The phenomenon of population interference, where a treatment assigned to one experimental unit affects another experimental unit’s outcome, has received considerable attention in standard randomized experiments. The complications produced by population interference in this setting are now readily recognized, and partial remedies are well known. Less understood is the impact of population interference in panel experiments where treatment is sequentially randomized in the population, and the outcomes are observed at each time step. This paper proposes a general framework for studying population interference in panel experiments and presents new finite population estimation and inference results. Our findings suggest that, under mild assumptions, the addition of a temporal dimension to an experiment alleviates some of the challenges of population interference for certain estimands. In contrast, we show that the presence of carryover effects — that is, when past treatments may affect future outcomes — exacerbates the problem. Our results are illustrated through both an empirical analysis and an extensive simulation study.

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Recommended citation: Kevin Han, Guillaume Basse, and Iavor Bojinov. Population interference in panel exper- iments. Journal of Econometrics, 238(1):105565, 2024.