Two groups, N1 = α1 + b1 and N2 = α2+ b2, are compared in terms of their respective morbidity rates, p1 and p2, in the presence of errors of misclassification (α1 and α2 are sure components; b1 and b2 are potential error terms). The four extreme situations for the compositions of N1 and N2 are considered. Rate p1 is assumed to apply to α1 and b1; p2 is assumed to apply to α1 and b2. If there is a difference in rates for the error-free situation, then the observed difference is smaller in each of the error situations. Further, measurement errors reduce the chance of detecting a real difference.