Paper-and-pencil questionnaires returned by post (postal questionnaires) have traditionally served as a primary data collection method in social science research. However, the use of computer-based questionnaires completed on the Internet (online questionnaires) is becoming commonplace. Despite their widespread use, the quality of online questionnaire data has not been comprehensively investigated. Data quality may be affected by both measurement and non-measurement error. Numerous within-subjects studies in which participants completed paper-and-pencil and computer-based versions of the same questionnaire have demonstrated the equivalency of data collected using both questionnaire modes. Therefore, measurement error does not appear to differentially affect the quality of data collected via computer. However, these studies do not take into account that use of different questionnaire modes may introduce non-measurement errors such as participant non-response, item non-response, and selection bias. Although several studies have compared the quality of data from telephone, face-to-face, and postal questionnaires, few studies have compared postal with online questionnaires.