1. C. F. Turner and S. M. Rogers are with the Program in Health and Behavior Measurement at the Research Triangle Institute, 1635 M Street, NW, Washington, DC 20036, USA. L. Ku, L. D. Lindberg, and F. L. Sonenstein are with the Urban Institute, 2100 M Street, NW, Washington, DC 20037, USA. J. H. Pleck is with the Department of Human Development and Community Studies, University of Illinois, Urbana, IL 61801, USA.