Effects of drop out in a longitudinal study of musculoskeletal disorders

Objectives-The drop out rates in different longitudinal studies of musculos keletal disorders range between 7% and 57%, and little is known about the c haracteristics of the subjects who dropped out. The aim was to analyse vari ous consequences of drop out in a longitudinal study of musculoskeletal dis orders and occupational risk factors during 1969-97. Method-Data about occupational conditions and health in 1969 and in 1993 we re analysed. Differences between those who participated throughout (partici pants) and drop out subjects in these analyses formed the basis for recalcu lations of earlier reported analyses of associations between occupational c onditions and low back pain. In the recalculation the data were weighted to compensate for the differences. Results-More female and male drop out subjects than participants in 1993 ha d monotonous work, fewer women and more male drop out subjects had heavy Li fting in 1969. In 1997, more female and male drop out subjects had had heav y lifting and low stimulation at work in 1993. At bath occasions, there wer e differences 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-Differences in occupational conditions and health among partici pants and drop out subjects in a longitudinal study of musculoskeletal diso rders and occupational risk factors during 1969-97 did not markedly influen ce the risk ratios.