1. Department of Occupational Medicine and Epidemiology, National Institute of Occupational Health, Oslo, Norway
2. Department of Chronic Diseases and Ageing, Norwegian Institute of Public Health, Oslo, Norway
3. The Ragnar Frisch Centre for Economic Research, Oslo, Norway
4. Epidemiology, Statistics, and Population Sciences, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, Maryland, USA.