The school board of a large school district would like to raise taxes in the district to pay for new computers. To see if there is support for a tax increase, they send a survey home with a random sample of students in the district. If the survey is not completed, calls are made to ensure responses are obtained from the parents of every selected student. The survey asks, "Would you be in favor of a modest tax increase to fund the purchase of new computers for all students?” The confidence interval for the true proportion of families that are in favor of a tax increase is 0.70 to 0.80. Members of the community who do not have children in the district complain about not being included in this study. Which of the following sources of bias affected the confidence interval, but is not included among the sources covered by the margin of error?