1. Office of Research and Methodology; National Center for Health Statistics, Centers for Disease Control and Prevention; Hyattsville MD 20782 U.S.A.
2. Office of Analysis and Epidemiology; National Center for Health Statistics, Centers for Disease Control and Prevention; Hyattsville MD 20782 U.S.A.
3. Department of Health Care Policy; Harvard Medical School; Boston MA 02115 U.S.A.