The sixth survey of the Tromso Study (Tromso 6) in 2007-08: collaborative research in the interface between clinical medicine and epidemiology: study objectives, design, data collection procedures, and attendance in a multipurpose population-based health survey - Eggen, A E; Mathiesen, E B; Wilsgaard, T; Jacobsen, B K; Njølstad, I (Scand J Public Health, 2013)