An apparently incongruous exposure-response relationship resulting from the use of job description to assess magnetic field exposure.

Empirical data are presented to support the observation that nondifferential misclassification of subjects classified according to an ordinal scale can result in an artifactual nonmonotonic exposure-response curve. This phenomenon can be interpreted erroneously as an indication that an observed relationship is not causal. The occupational exposure of 40 subjects to extremely low-frequency magnetic fields was estimated on the basis of their job description while their "true" exposure was determined by direct dosimetry. A "true" risk was assumed to exist, and estimates of risk that could be obtained through hypothetical case-referent or cohort studies were calculated.