Two groups,N1= α1+b1andN2= α2+b2, are compared in terms of their respective morbidity rates,p1andp2, in the presence of errors of misclassification (α1and α2are sure components;b1andb2are potential error terms). The four extreme situations for the compositions ofN1andN2are considered. Ratep1is assumed to apply to α1andb1;p2is assumed to apply to α1andb2. 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.