It is common practice to assume that inspectors of sample items and correctors of items in rejected work lots carry out their job without error and invariably classify defectives and nondefectives correctly. This assumption does not agree with experience in data processing activities, especially those requiring interpretation and judgment as well as clerical or machine skills. This paper emphasizes the error of classifying defectives as nondefectives (Type II) made by inspectors called “verifiers” and by correctors to prepare data for machine readability. The following activities are included: (1) verification of card punching, (2) proofreading of typed data, (3) verification of codes clerically assigned to written responses on questionnaires, (4) matching of codes assigned independently by three coders to the same item, and (5) proofreading of the output of special typing combined with coding and rounding figures for input to optical scanning.