Why are reminders sent to respondents who have already completed the survey?
Due to initial security and confidentiality features, everyone surveyed will receive reminders, even those who have already responded. When submitted, survey responses and identifying information are immediately separated, so we have no way of knowing who has already responded. Reminders, therefore, are distributed to everyone in the survey group.
Reminders are also sent because research indicates that the single highest predictor of response rates in web-based surveys is the number of contacts made, including reminders. (See: Cook, Heath, and Thompson, A meta-analysis of response rates in web or internet-based surveys, Educational and Psychological Measurement, v. 60, 2000, p.821-836.)