1. Department of Epidemiology and Biostatistics, University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, Texas 78229, U.S.A. choib@uthscsa.edu
2. Department of Biostatistics, University of North Carolina, 3103B McGavran-Greenberg Hall, Chapel Hill, North Carolina 27599, U.S.A. jfine@email.unc.edu
3. Department of Epidemiology, University of North Carolina, 2105F McGavran-Greenberg Hall, Chapel Hill, North Carolina 27599, U.S.A. mabrook@email.unc.edu