1. Norwegian Institute of Public Health; Division of Health Services; Marcus Thranes gate 6 Oslo Norway 0403
2. University of Toronto; Department of Family and Community Medicine; 500 University Avenue Fifth Floor Toronto ON Canada M5G 1V7
3. Lazio Regional Health Service - ASL Roma1; Department of Epidemiology; Rome Italy
4. Ottawa Hospital Research Institute; Clinical Epidemiology Program; The Ottawa Hospital - General Campus 501 Smyth Road, Box 711 Ottawa ON Canada K1H 8L6