Population Interference in Panel Experiments

TL;DR

This paper introduces a framework for understanding population interference in panel experiments, revealing how temporal data can mitigate some interference issues but complicate others like carryover effects.

Contribution

It provides new finite population estimation methods and theoretical results for panel experiments with population interference, extending understanding beyond standard experiments.

Findings

Temporal dimension can reduce interference challenges for some estimands

Carryover effects worsen the impact of population interference

Central limit theorem established under weaker conditions

Abstract

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. Much 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. Revisiting the special case of standard experiments with population interference, we prove a central limit theorem under weaker conditions than previous results in the literature and highlight the trade-off between flexibility in the design and the interference structure.