1. Department of Orthopaedics; Faculty of Medicine Ramathibodi Hospital, Mahidol University; 270 Rama VI Road, Ratchathewee Bangkok 10400 Thailand
2. Department of Preventive and Social Medicine; Faculty of Medicine, Chulalongkorn University; Bangkok 10330 Thailand
3. Centre for Clinical Epidemiology and Biostatistics, The University of Newcastle; Newcastle NSW Australia
4. Section for Clinical Epidemiology and Biostatistics; Faculty of Medicine Ramathibodi Hospital, Mahidol University; Bangkok 10400 Thailand