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This study was done in preparation for the launch of the National Children's Study (NCS) main study. The goal of this study was to examine the feasibility (completion rates and completeness of data), acceptability, staff time and cost-effectiveness of three methods of data collection for the postnatal 3- and 9-month questionnaires completed as part of NCS protocol. Eligible NCS participants who were scheduled to complete a postnatal questionnaire at three and nine months were randomly assigned to receive either: (a) telephone data collection (b) web-based data collection, or (c) self-administered (mailed) questionnaires. Event completion rates and satisfaction across the three data collection methods were compared and the influence of socio-demographic factors on completion rates and satisfaction rates was examined. Cost data were compared to data for completion and satisfaction for each of the delivery methods. Completion rates and satisfaction did not differ significantly by method, but completeness of data did, with odds of data completeness higher among web than phone (p < 0.001) or mail (p < 0.001). Costs were highest for the phone, followed by mail and web methods (p < 0.001). No significant differences in participant time (i.e. burden) across the three data collection methods were seen. Mail and phone data collection were the least complete of the three methods and were the most expensive. Mailed data collection was neither complete nor exceptionally economical. Web-based data collection was the least costly and provided the most complete data. Participants without web access could complete the questionnaire over the phone.[PUBLICATION ABSTRACT]