The accuracy of procedures that are used to compare the performance of different groups of examinees on test items obviously depends on the correct classification of members in each examinee group. The significance of this dependence is determined by the sensitivity of the statistical procedure and the proportion of examinees who are unidentified. Since the number of nonrespondents to questions of ethnicity is often of the same order of magnitude as the number of identified members of most minority groups, understanding the effect of nonresponse is crucial to evaluating the validity of a procedure that is used to study differential item functioning (DIF). This study examined the effect of nonresponse to questions of ethnic identity on the measurement of DIF for Scholastic Aptitude Test verbal items using a commonly used modern method, the Mantel-Haenszel procedure (P. W. Holland and D. T. Thayer, 1988). The study considered 94,486 examinees who identified themselves as black or white or did not identify themselves by ethnic group. It was found that efforts to obtain more complete ethnic identifications from the examinees would be rewarded with more accurate DIF analyses. (Contains four tables and eight references.) (Author/SLD)