A homeroom was interested in surveying their students to find out opinions about the schools media center. To facilitate data collection, the homeroom period was extended 30 minutes to allow everyone in the school ample time to respond to a short questionnaire. Unfortunately, it was learned after the surveys had been completed that all honors students in the middle school were on an all-day field trip and away from school for the entire day. The exclusion of their input into the survey would be considered which type of sampling problem?