A large school district is considering changing the start time of their elementary and high schools. The superintendent wants to poll parents to get their feedback on the proposed change. Some parents in the district have their email address on file with the district, so the superintendent emails a survey to a random sample of addresses. After a week, they send a reminder email to those addresses that encourages parents to fill out the survey if they haven't already.
Which of the following is not a potential source of bias in their survey?