https://macropaperwarehouse.com/authors/alexandre-poirier/Alexandre PoirierHugo -- gohugo.ioen-ushttps://macropaperwarehouse.com/papers/the-effect-of-omitted-variables-on-the-sign-of-regression-coefficients/https://macropaperwarehouse.com/papers/the-effect-of-omitted-variables-on-the-sign-of-regression-coefficients/<p>Masten and Poirier demonstrate a previously unrecognized asymmetry in the coefficient stability literature: depending on how omitted variable bias is measured, it can be substantially easier for omitted variables to flip a regression coefficient&rsquo;s sign than to drive it to zero. The paper focuses specifically on Oster (2019b), a widely used robustness framework with approximately 5,500 Google Scholar citations as of December 2025, and shows that Oster&rsquo;s sensitivity parameter δ — commonly interpreted as the ratio of selection on unobservables to selection on observables — exhibits a structural problem when used to assess sign robustness.</p>