[1] Mahidol Univ, Ramathibodi Hosp, Fac Med, Sect Clin Epidemiol & Biostat, Bangkok 10400, Thailand
[2] Thammasat Univ, Dept Surg, Fac Med, Thammasat Univ Hosp, Pathum Thani, Thailand
[3] Chonburi Hosp, Dept Surg, Chon Buri, Thailand
[4] Mahidol Univ, Ramathibodi Hosp, Fac Med, Dept Surg, Bangkok 10400, Thailand
[5] Univ Newcastle, Ctr Clin Epidemiol & Biostat, Newcastle, NSW 2300, Australia