1. Section of Social Medicine, Department of Public Health, University of Copenhagen, Øster Farimagsgade 5, DK-1014 Copenhagen K, Denmark
2. Back- and Rehabilitation Centre Copenhagen, Mimersgade 41, DK-2200 Copenhagen N, Denmark
3. Department of Occupational Health, Copenhagen University Hospital Holbæk, Smedelundsgade 60, 4300 Holbæk, Denmark