[1] Natl Univ Singapore, Dept Stat & Appl Probabil, Singapore 117543, Singapore
[2] NHLBI, Div Epidemiol & Clin Applicat, Off Biostat Res, Bethesda, MD 20892 USA
[3] George Washington Univ, Dept Stat, Washington, DC USA
[4] NCI, Biostat Branch, Div Canc Epidemiol & Genet, Dept Hlth & Human Serv,NIH, Washington, DC USA