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A high school has a policy that students' phones must be kept away during class. A principal used the school roster to poll a random sample of 50 students, and only 10% said that they ever had their phone out during class. The next day, the principal observed classrooms and noticed that approximately 25% of students had their phone out at some point during class.
Which of these is the most concerning potential source of bias in the principal's poll?
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