The authors of the paper "Digital Inequality: Differences in Young Adults' Use of the Internet" (Communication Research [2008]: 602-621) were interested in determining if people with higher levels of education use the Internet in different ways than those who do not have as much formal education. To answer this question, they used data from a national telephone survey. Approximately 1300 households were selected for the survey, and 270 of them completed the interview. What type of bias should the researchers be concerned about and why?