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The pervasiveness of the Internet represents an unprecedented opportunity to devise potentially massive and relatively inexpensive epidemiological studies. Voluntary participation, as well as recruitment through a dedicated web site, is easy to manage and advertise. In the last few years we have witnessed increasing interest in Internet-based study design, including in methodological research on how to use the Internet to recruit and follow-up participants in cohort studies.
Enrollment of a convenience sample using the Internet restricts the source population (to Internet users) and generates a self-selected study population. In this paper we will not discuss issues of validity related to this baseline selection but we will study follow-up participation and, in particular, whether the different strategies to communicate and advertise the existence of the study and to enroll participants are related to compliance with subsequent follow-up.