“…A chief residency is demanding and has substantial responsibilities, as noted by Drs Silverstein, Stroud, and Zipursky, and chiefs hit the ground running on day 1. If our study findings entirely reflected a treatment effect, we would have to believe that the selection process fails to identify skilled residents or that nonchiefs do not catch up as they practice. Importantly, the counterfactual of a chief residency is likely becoming an attending physician a year earlier, which surely contributes to the physician’s development.…”