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ders, the initial list of 265 references was reduced to 13 that provided direct and 29 that provided indirect measures of exposure. For psychophysical factors and upper extremity disorders, the initial 120 references were reduced to 28.

Analysis of Study Results

Definition of Measures: Relative Risks

In epidemiology, the relative risk is a measure of the strength of an association, here meaning the relationship between the frequency of an exposure and the occurrence of an outcome (e.g., amount of vibration and incidence of back pain). Because human populations typically have a variety of exposures occurring in near proximity, relative risk is typically measured as the incidence of disease in the exposed (e.g., helicopter pilots who experience vibration) and the incidence of disease in the unexposed (similar people, like ground crews, who are considered to share nearly the same other exposures as the exposed, such as recreational activities, diet, and living conditions). The ratio of incidence provides a measure of association, and the higher this ratio of incidences (the relative risk), the stronger the association, the more confidence we can place in a conclusion that the association is meaningful.

Because incidence is a rate calculated by following people over time, and many studies are cross-sectional or retrospective (case-control), other measures, such as the prevalence ratio and the odds ratio, have been developed to summarize the association between exposure and outcomes for these other study designs. Our analysis focused on associations expressed by such risk estimates as the odds ratio and the relative risk. These estimates were retrieved from the original article or calculated when sufficient raw data were presented.

Definition of Measure: Attributable Risk

The attributable risk is another measure used to help generate inferences. In its simplest form, it is the difference between the incidence in those exposed and those unexposed—a risk difference. This risk difference is thought of as the attributable risk in that, in theory, removing this exposure entirely would reduce the frequency of the outcome to the level of those who are unexposed. Rotham and Greenland (1998a, 1998b) discuss some of the limitations of this simple assumption. Attributable risk is often calculated as a ratio rather than a difference: risk in the exposed is divided by risk in the unexposed, producing an attributable fraction. The attributable fraction is the proportion by which the rate of the outcome



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