© 2008 American Thoracic Society
Effects of Misclassification of Fume ExposureTo the Editor:In their discussion of the effect of fume and dust exposure on the decline of pulmonary function in early chronic obstructive pulmonary disease (COPD), Dr. Harber and colleagues (1) comment on the relatively crude metric of self-report of exposure to fume. They conjecture that the regression coefficient may have underestimated the actual effect, since "any misclassification error is likely to bias toward the null." There is an important qualifier to this principle, namely, that while nondifferential misclassification often does introduce bias toward the null, differential misclassification can either exaggerate or underestimate an effect. With nondifferential misclassification, the misclassification error is independent of other variables; with differential misclassification, the error depends on the value of other variables (2). In the population under study, subjects with early COPD reported on their own occupational fume exposure. If persons with more rapidly progressing disease or with diminished physiologic reserve were more vigilant about potentially harmful environmental variables, they might report fume exposure more completely than persons with less severe disease. If this occurred, the classification error would be associated with disease status and would be differential misclassification. The regression coefficient could exaggerate the actual effect. Self-report of exposure may easily be influenced by disease status. A caveat about the magnitude of the regression coefficient for fume exposure is therefore warranted.
Colorado Division of Workers' Compensation FOOTNOTES Conflict of Interest Statement: E.W. has no financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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