1. Zhiqiang Tan is Assistant Professor, Department of Biostatistics, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD 21205 . This research is supported by a Faculty Innovation Fund from the School. The author thanks Constantine Frangakis, Tom Louis, and Dan Scharfstein for stimulating discussions, David Card for kindly sharing the data, and the editor, an associate editor, and two referees for helpful comments.