- Surveillance of work-related cumulative trauma disorder of the upper extremity (CTDUE) requires valid and reliable claim extraction strategies and should examine for confounding and interaction. This research estimated crude and specific rates of CTDUE claims in Ontario workers during 1997 while acknowledging misclassification and testing for confounding and interaction. Lower and upper limit event estimates were obtained by means of an algorithm applied to the Ontario Workplace Safety and Insurance Board (OWSIB) database and were combined with "at-risk" estimates obtained from the Canadian Labour Force Survey (LFS). Poisson regression was used to evaluate confounding and interaction. The method used to identify CTDUE claims had a substantial impact on the magnitude of rates, female to male rate ratios, the most commonly affected part of the upper extremity and the highest risk occupational categories. Poisson regression identified sex interactions. It allowed rigorous evaluation of the data and indicated that rates should be examined separately for men and women. Researchers should clearly define extraction strategies and examine the impact of misclassification.