Does the fact that only a third of the respondents answering a mail questionnaire answer all items reduce substantially the reliability of the results? This is a study of 14,600 usable questionnaires from a 40,000 mailing. It analyzes item nonresponses by characteristics of respondents and by type, position, and repetition of questions. Robert Ferber is Director of the Survey Research Laboratory and Research Professor of Economics and Marketing at the University of Illinois, and Editor of the Journal of Marketing Research.