1. 1 U.S. Census Bureau, Research and Methodology , 4600 Silver Hill Road, Suitland, Maryland, 20746 U.S.A .
2. 2 University of Michigan , Department of Biostatistics, School of Public Health, University of Michigan , M4041 SPH II, 1420 Washington Heights, Ann Arbor, Michigan 48109, Michigan, 48105 U.S.A .