Information bias in a case-referent study on mental retardation and parental occupation: colleagues as dual respondents.

We conducted a validity study to assess whether information bias from differential misclassification of occupational exposures occurred in a case-referent study on mental retardation and parental occupation. The colleagues of 42 case parents and 38 referent parents were traced and interviewed with a questionnaire similar to that used for the parents. Within each parent-colleague pair, Cohen's kappa coefficient was calculated as a measure of agreement between the reported occupational exposures. Averaged across pairs, the mean kappa coefficient was 0.48 (95% CI = 0.40-0.56) for case pairs and 0.46 (95% CI = 0.38-0.54) for referent pairs, indicating little evidence for differential reporting of exposures between cases and referents. Our results also indicate that colleagues are eligible substitute respondents for information on occupational exposures.