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Which Policy Works, and Where? Estimation and Inference for State-Level Treatment Effects in Difference-in-Differences

Nichole Austin, Sunny R. Karim, Erin Strumpf, Matthew D. Webb

arXiv 1 Sep 2026 · Econometrics

arXiv:2609.01467 · PDF · Extracted main text

Abstract

Policies with a common objective and implementation date may differ in details or context. We distinguish the aggregate average treatment effect on the treated (ATT) from sub-aggregate ATTs defined by implementation cohort, jurisdiction, period, or policy type. UN-DID and DID-INT, two DiD estimators that construct jurisdiction-by-time effects, estimate these ATTs under parallel-trends conditions matched to the aggregation. In CPS placebo-law simulations, randomization inference is generally well-sized, though some jurisdiction-specific tests are conservative. The jackknife can be undefined for sub-aggregate ATTs; when defined, it over-rejects with few treated or comparison jurisdictions. Estimands and inference methods should match the policy question and implementation setting.

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24
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Most heavily cited references

The works this paper leans on most, across its whole bibliography — not restricted to papers in our corpus. Ranked by composite intensity, which combines how often a work is mentioned, how many sections mention it, and how much of that falls in the main text rather than the appendix.

ReferenceIntensityMentionsSectionsMain text
1Callaway, Brantly and Sant'Anna, Pedro H. C (2021) Difference-in-Differences with Multiple Time Periods1.00064100%
2MacKinnon, James G. and Webb, Matthew D (2020) Randomization Inference for Difference-in-Differences with Few Treated Clusters self0.92843100%
3MacKinnon, James G. and Nielsen, Morten Ø. and Webb, Matthew D (2023) Fast and Reliable Jackknife and Bootstrap Methods for Cluster-Robust Inference self0.87452100%
4Karim, Sunny and Webb, Matthew D (2024) Good Controls Gone Bad: Difference-in-Differences with Covariates self0.87452100%
5Karim, Sunny and Nielsen, Morten Ørregaard and MacKinnon, James G. a… (2026) Improved Inference for CSDID Using the Cluster Jackknife self0.84333100%
6Goodman-Bacon, Andrew (2021) Difference-in-Differences with Variation in Treatment Timing0.81142100%
7Karim, Sunny and Webb, Matthew D. and Austin, Nichole and Strumpf, E… (2024) Difference-in-Differences with Unpoolable Data self0.81142100%
8MacKinnon, James G. and Nielsen, Morten Ø. and Webb, Matthew D (2023) Cluster-Robust Inference: A Guide to Empirical Practice self0.64422100%
9Bertrand, Marianne and Duflo, Esther and Mullainathan, Sendhil (2004) How Much Should We Trust Differences-in-Differences Estimates?0.64422100%
10Conley, Timothy G. and Taber, Christopher R (2011) Inference with “Difference in Differences” with a Small Number of Policy Changes0.64422100%

Showing the top 10 of 24 scored citations.