Real-time Program Evaluation using Anytime-valid Rank Tests

Counterfactual mean estimators such as difference-in-differences and synthetic control have grown into workhorse tools for program evaluation. Inference for these estimators is well-developed in settings where all post-treatment data is available at the time of analysis. However, in settings where data arrives sequentially, these tests do not permit real-time inference, as they require a pre-specified sample size T. We introduce real-time inference for program evaluation through anytime-valid rank tests. Our methodology relies on interpreting the absence of a treatment effect as exchangeability of the treatment estimates. We then convert these treatment estimates into sequential ranks, and construct optimal finite-sample valid sequential tests for exchangeability. We illustrate our methods in the context of difference-in-differences and synthetic control. In simulations, they control size even under mild exchangeability violations. While our methods suffer slight power loss at T, they allow for early rejection (before T) and preserve the ability to reject later (after T).