1. Center for Causal Inference, University of Pennsylvania, Philadelphia, Pennsylvania, U.S.A
2. Department of Applied Economics and Statistics, University of Delaware, Newark, Delaware, U.S.A
3. Department of Applied Mathematics and Statistics, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland, U.S.A
4. Department of Biomedical Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland, U.S.A