1. University of Michigan - Survey Research Center, Ann Arbor, MI 48106-1248, U.S.A.
2. Westat, 1600 Research Boulevard, Rockville, MD 20850, U.S.A.
3. University of Michigan - School of Public Health, Dept. of Biost., M4041 SPH II, 1420 Washington Heights, Ann Arbor, MI 48105, U.S.A.