1. From the Framingham Heart Study (J.M.M., R.B.D., H.S., P.W.F.W.) Framingham, Mass; Section of Preventive Medicine and Epidemiology (J.M.M.) and Section of General Internal Medicine (J.M.M.) of Boston University School of Medicine, Boston, Mass; Division of Epidemiology and Clinical Applications of the National Heart, Lung, and Blood Institute (P.W.F.W.), Bethesda, Md; and the Department of Mathematics of Boston University (R.B.D., H.S.), Boston, Mass.