@TechReport{iza:izadps:dp18274,
            	author={Bjerk, David J.},
            	title={Understanding IV Versus OLS Estimates of Treatment Effects and the Coefficient Difference Check},
            	year={2025},
	month={Nov},
            	institution={Institute of Labor Economics (IZA)},
            	address={Bonn},
            	type={IZA Discussion Paper},
            	number={18274},
            	url={https://www.iza.org/publications/dp18274},
	abstract={This article derives an equation characterizing the difference between OLS and IV coefficients under potentially heterogenous treatment effects. This leads to what I call the Coefficient Difference Check, which consists of checking that the difference between the estimated OLS and IV coefficients has the same sign as the expected selection effect. I show failures of this check can arise because: IV is invalid, the expected selection story is incorrect, or there are particular heterogenous treatment effects that imply the IV estimate is both “fragile” and that it provides a more biased estimate of the ATT than OLS. Failures of this check are relatively common in the literature. I describe best practices given such failures.},
	keywords={adjudicator propensity to treat IV;judge fixed-effects;average treatment-on-the-treated;heterogenous treatment effects;selection;instrumental variables;examiner tendency IV;returns to schooling},
            }