A Cautionary Tale About Control Variables in IV Estimation
讨论了工具变量回归中控制变量的时机问题，指出若控制变量在工具分配后测量，条件独立性可能不成立，并通过越战征兵风险作为工具的例子，说明使用不当控制变量会导致严重偏误。
Abstract 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.