EVects of drop out in a longitudinal study of musculoskeletal disorders

Objectives—The drop out rates in diVerent longitudinal studies of musculoskeletal disorders range between 7% and 57%, and little is known about the characteristics of the subjects who dropped out. The aim was to analyse various consequences of drop out in a longitudinal study of musculoskeletal disorders and occupational risk factors during 1969–97. Method—Data about occupational conditions and health in 1969 and in 1993 were analysed. DiVerences between those who participated throughout (participants) and drop out subjects in these analyses formed the basis for recalculations of earlier reported analyses of associations between occupational conditions and low back pain. In the recalculation the data were weighted to compensate for the diVerences. Results—More female and male drop out subjects than participants in 1993 had monotonous work, fewer women and more male drop out subjects had heavy lifting in 1969. In 1997, more female and male drop out subjects had had heavy lifting and low stimulation at work in 1993. At both occasions, there were diVerences between the drop out subjects and participants in occurrence of musculoskeletal disorders. The weighted analyses resulted in changes in risk ratio of 0.1–0.2. Conclusions—DiVerences in occupational conditions and health among participants and drop out subjects in a longitudinal study of musculoskeletal disorders and occupational risk factors during 1969–97 did not markedly influence the risk ratios. (Occup Environ Med 2001;58:194–199)