The Perils of Counterfactual Analysis with Integrated Processes
41 Pages Posted: 6 Jan 2017
Date Written: December 27, 2016
Recently, there has been a growing interest in developing econometric tools to conduct counterfactual analysis with aggregate data when a “treated” unit suffers an intervention, such as a policy change, and there is no obvious control group. Usually, the proposed methods are based on the construction of an artificial counterfactual from a pool of “untreated” peers, organized in a panel data structure. In this paper, we investigate the consequences of applying such methodologies when the data are formed by integrated process of order 1. We find that without a cointegration relation (spurious case) the intervention estimator diverges resulting in the rejection of the hypothesis of no intervention effect regardless of its existence. Whereas, for the case when at least one cointegration relation exists, we have a √ T-consistent estimator for the intervention effect albeit with a non-standard distribution. However, even in this case, the test of no intervention effect is extremely oversized if nonstationarity is ignored. When a drift is present in the data generating processes, the estimator for both cases (cointegrated and spurious) either diverges or is not well defined asymptotically. As a final recommendation we suggest to work in first-differences to avoid spurious results.
Keywords: Counterfactual Analysis, Comparative Studies, Panel Data, ArCo, Synthetic Control, Policy Evaluation, Intervention, Cointegration, Factor Models, Spurious Regression, Nonstationarity
JEL Classification: C22, C23, C32, C33
Suggested Citation: Suggested Citation