criteria, and included PE maneuvers such as tenderness to palpation and pain on resisted movement, as well as examination for other causes of upper extremity symptoms.

For carpal tunnel syndrome, the use of case definitions based on symptoms and physical examination alone is more controversial. The most accurate definition of CTS would include electrodiagnostic studies (EDS) as well as symptoms (and possibly PE). This definition was used in some of the studies reviewed by NIOSH. EDS alone should not be used as a case definition. Several studies have evaluated the performance of case definitions based on symptoms and physical examination alone; although there is some misclassification of disease using this case definition, it is appropriate for studies where the disadvantage of misclassification can be offset by other factors, such as a larger sample size.

The weighted studies did not use self-report or job titles as a measure of exposure, but used directly observed and measured data on specific exposures.

The weighted studies controlled for potential biases and confounders through a variety of mechanisms. Individual risk factors such as age, gender, pre-existing disease, non-occupational activities, and metabolic diseases were controlled to some extent in all the weighted studies. Measurements of health and exposure status were done in a blinded manner. Weighted studies all had participation rates of 370%, decreasing the chance of respondent bias.

3. Would either the inclusion of any omitted studies or the assessment of the quality of those reviewed substantially alter the interpretation of the epidemiological evidence that certain physical stressors in the workplace increase risk of acquiring certain MSDs?

Addition of omitted studies would not alter the interpretation of the evidence. More stringent requirements for study quality could change the conclusions only by excluding most or all current studies.

The literature review and study weighting process adopted by NIOSH included most or all important evidence. Other authors, using different criteria for literature selection, have also concluded that certain physical stressors in the workplace increase the risks of acquiring certain MSDs. Authors who conclude that there is insufficient evidence to permit this interpretation have excluded most or all of the current epidemiological evidence, citing the lack of longitudinal data and questions about the significance of health outcomes measured in these studies.

The well-designed cross sectional studies weighted by NIOSH provide data useful for causal inference. These studies took steps to avoid bias in the measurement of exposures and health outcomes. Data on the temporality of exposure and symptom or disease onset were available to ensure that the putative exposures occurred before symptom onset. These studies thus offer more than a "snapshot in time." Other potential causes of the observed associations were sought. To discount these studies, one must assume that measurements were significantly biased, or that persons prone to developing MSDs were preferentially placed into jobs with high physical demands. In cross-sectional studies, survivor effects will typically decrease the observed associations between symptomatic disorders and physically demanding jobs.



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