Problems with cross-sectional data in research on working environment and health.

Occupational turnover due to health related selection might introduce a bias in cross-sectional studies that tends to mask real occupational health effects. People could have changed occupation so that they, when disease occurs and/or the data collection is accomplished, are working in an environment that is irrelevant for the disease in question. The aim of this study was to determine whether there is any difference in morbidity between 'stable' workers and 'changers'. Occurrence of long-term illness was studied on four exposure levels, defined according to the physical demands at work. The study populations comprised 10,487 men and 10,058 women between 25 to 74 years of age, who were interviewed within the scope of the Statistics Sweden Survey of Living Conditions in the years 1977 and 1979-81. Considerable differences in health outcomes were found between stable workers and changers, especially when considering the degree of physical strain at work. Thus, the findings indicate the necessity of detailed recording of occupational histories within the framework of cross-sectional studies, especially when the aim of the investigation is to study and compare health outcomes for workers in occupations with different turnover rates.