Bias due to non-differential misclassification of polytomous confounders

This paper addresses potential effects of non-differential misclassification of polytomous confounders on adjusted exposure-disease associations. Although the degree of confounder-misclassification bias heavily depends on the relative distribution of the confounding variable among the compared exposure groups and the misclassification pattern, in most cases the bias is in the same direction (though to a lesser degree) than the confounding, i.e. the observed adjusted measures lie between the crude and the fully adjusted measures. In some instances, however, the confounder misclassification bias may be in the opposite direction. This is in contrast to previous understanding that non-differential confounder misclassification always tends to bias adjusted effect estimates towards the crude estimates and that the extent of this bias has a stable relationship to the degree of misclassification. Consequently, conclusions on the potential effects of non-differential misclassification of a polytomous confounder in any given study should only be made after careful sensitivity analyses which consider plausible ranges of misclassification rates.