How Robust are Robustness Checks?

Robustness checks are routine in empirical work, but there is no standard statistical procedure to formally measure what one can learn from them. I propose a "robustness radius" measure to quantify the amount by which the robustness checks estimands differ from the main specification estimand. I do so by framing robustness checks as explicitly biased regressions, clarifying what exactly the estimands are when comparing multiple regressions with slightly different samples, and applying a test from the moment inequalities literature. The robustness radius is easily interpretable and adapts to sampling uncertainty and correlation across regressions. An application shows that, although assessing overall robustness is context-specific, the robustness radius guides those judgments and improves transparency.