A study examined the error rate of the "University of California Union Catalog Supplement," a 47 volume, computer produced book catalog of the materials cataloged by the nine University of California campuses during the years 1963-67. The study attempted not only to determine the rate and nature of errors, but also to develop a methodology for studying large bibliographic files. A stratified sample of 94 pages was examined, types and rates of errors were identified, and errors were categorized according to degree of seriousness. This report describes the methodology and provides tabular summaries and explanations of the results. (EMH)