A0483
Title: Estimating the causal effect of policy interventions in the presence of spillovers
Authors: Nandita Mitra - University of Pennsylvania (United States) [presenting]
Abstract: Public policy interventions are commonly evaluated using the difference-in-differences approach. However, this approach does not directly account for the effect of the policy ``spilling over` to neighboring regions such as nearby cities or states. For example, the implementation of an excise tax on sugar-sweetened and artificially sweetened beverages in the city of Philadelphia was shown to be associated with a substantial decrease in volume sales of taxed beverages in Philadelphia but also showed an increase in beverage volume sales in bordering counties which were not subject to the excise tax. The latter association could potentially be explained by cross-border shopping behaviors of Philadelphia city residents. To address these important concerns, we extend difference-in-differences methods to identify the causal effects of policy interventions under various spillover conditions. We propose doubly robust estimators for the average treatment effect on the treated and on the neighboring control. The new estimators relax the standard assumptions on interference and model specification. In addition, we formally define a new causal estimator for the average treatment effect on the treated as a function of neighborhood exposure to the policy intervention. Importantly, our approach allows one to generalize the causal effect of a policy change to other target populations that may be different from the original study population.
