TY  - RPRT
        AU  - Bjerk, David J.
        TI  - Understanding IV Versus OLS Estimates of Treatment Effects and the Coefficient Difference Check
        PY  - 2025/Nov/
        PB  - Institute of Labor Economics (IZA)
        CY  - Bonn
        T2  - IZA Discussion Paper
        IS  - 18274
        UR  - https://www.iza.org/index.php/publications/dp18274
        AB  - 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.
        KW  - adjudicator propensity to treat IV
KW  - judge fixed-effects
KW  - average treatment-on-the-treated
KW  - heterogenous treatment effects
KW  - selection
KW  - instrumental variables
KW  - examiner tendency IV
KW  - returns to schooling
        ER  -