Bias in risk estimates from variability of exposure to postural load on the back in occupational groups.

Variability of exposure can be a source of information bias in studies with exposure assessment based on the use of a sample of workers in each occupational group under study. This paper presents a method to assess the rate of exposure misclassification from the magnitude of overlap of exposure distributions and, consequently, to evaluate the bias to risk estimates in cross-sectional and prospective studies. The percentage of work-time with trunk flexion and rotation was studied in five occupational groups. The rate of misclassification of exposure to trunk flexion and rotation varied from 0.03 to 0.35. Misclassification below 0.10 was found only for occupational groups with at least a 14-fold difference in mean exposure. Higher rates of misclassification can easily bias the risk estimates up to 50%. In the cross-sectional design the odds ratio was more sensitive to bias than the prevalence rate ratio. The estimate of the relative risk in a prospective study design was the least biased.