A Cautionary Tale About Control Variables in IV Estimation

Many instrumental variable (IV) regressions include control variables to justify (conditional) independence of the instrument and the potential outcomes. The plausibility of conditional IV independence crucially depends on the timing when the control variables are determined. This paper works through different IV models and discusses the (conditions for the) satisfaction of conditional IV independence depending on whether the control variables are measured prior to or after instrument assignment. To illustrate the identification issues, we consider the Vietnam War draft risk as instrument either for veteran status or education to evaluate the effects of these variables on labour market and health outcomes. Our empirical analysis based on the ‘Young Men in High School and Beyond’ survey suggests that commonly used conditional IV strategies to estimate the impact of draft induced military service or education may be severely biased due to the use of improper controls.