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The effect of joint misclassification of polytomous risk factors and polytomous covariates in case-control studies upon estimation and power has been examined using computer simulation and mathematical models. It is concluded that non-differential misclassification of covariates may be as important as non-differential misclassification of exposure variables, and that the resultant bias of the relative risk estimate may be either toward or away from the null. It is pointed out that when a confounding variable is misclassified the calculated p value from the particular study will be incorrect and may either be too large or too small depending on the nature of the confounding.
Fung et al. (Sun,) studied this question.