Fang et al. 2003), including misclassification (i.e., highly exposed individuals underreporting exposure and vice versa), healthy worker effect (i.e., highly exposed individuals who developed health outcomes avoiding further contact with the agent), and true biological leveling off in the highly exposed group (i.e., the adverse health consequence of the exposures saturates at a specific level). Finally, statistical power to identify associations was low. However, for a disease as rare as ALS, the number of cases available for study is typically limiting, and few studies have sufficient statistical power to detect the moderate associations usually reported for occupational exposures. In the future, it may be possible to combine results from several studies to increase power. Conclusions The present study suggests that ALS risk may be elevated in construction and precision metalworkers. 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