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Almost 60% of American households were connected to the Internet in 2001, when the Millennium Cohort Study, the largest longitudinal study ever undertaken by the Department of Defense, was launched. To facilitate survey completion, increase data integrity, and encourage cohort retention while maintaining the highest standards of participant privacy, an online questionnaire was made available on the World Wide Web in addition to a traditional paper questionnaire sent via US mail. Over 50% of 77,047 participants chose to enroll in the study via the Web, affording substantial cost savings to the project. Using multivariable logistic regression, the authors compared the demographic and health characteristics of Web responders with those of paper responders. Web responders were slightly more likely to be male, to be younger, to have a high school diploma or college degree, and to work in information …