Ancillary Studies of Experiments: Opportunities and Challenges

“Ancillary studies of experiments” are a technique whereby researchers use an experiment conducted by others to recover causal estimates of a randomized intervention on new outcomes. The method requires pairing randomized treatments the researchers did not oversee with data on outcomes that were not the focus of the original experiment. Since ancillary studies rely on interventions that have already been undertaken, oftentimes by governments, they can provide a low-cost method with which to identify effects on a wide variety of outcomes. We define this technique, identify the small but growing universe of papers that employ ancillary studies of experiments in political science and economics, and assess the benefits and limitations of the method.