Minimum Specification Perturbation: Robustness as Distance-to-Falsification in Causal Inference

TL;DR

This paper introduces Minimum Specification Perturbation (MSP), a new robustness measure in causal inference that quantifies how many analyst decisions must change to invalidate a causal claim, providing a distance-to-falsification metric.

Contribution

The paper proposes MSP as a novel robustness tool that captures the minimal decision changes needed to falsify causal results, outperforming dispersion-based summaries especially under weak effects.

Findings

MSP is small under the null hypothesis and increases with effect strength.

MSP provides a new distance-to-falsification measure not captured by existing dispersion summaries.

On the LaLonde benchmark, a single decision change can nullify the causal effect estimate.

Abstract

Empirical causal claims depend on many analyst decisions, from selecting covariates to choosing estimators. Existing robustness tools summarize how results vary across these choices, but, to the best of our knowledge, do not answer: \textbf{How many analyst decisions must change to reach a specification, which is a set of choices, whose confidence interval (CI) contains zero?} We introduce \emph{Minimum Specification Perturbation (MSP)}, the smallest number of changes. MSP is small under the null, grows with effect strength and captures distance-to-falsification information that dispersion-based summaries cannot report; when making decisions under weak effects, an MSP-based rule yields lower false-positive rates than dispersion-based rules. We show that Fragility Index and MSP measure orthogonal vulnerabilities: fragility to influential observations need not imply fragility to specification choices. On the LaLonde benchmark, MSP = 1 implies that one decision change makes the CI contain zero. We further provide exact permutation calibration under randomization and characterize computation, showing tractable cases under additive structure and NP-hardness in general.