Suppose a college has asked you to conduct a survey to determine the percentage of 8:00 AM classrooms that were full on a given morning. The college has three classroom buildings, each containing two lecture halls. Each lecture hall has a capacity of 100 students. You randomly choose one of three buildings, and stand outside the entrance when classes let out. You ask the first 60 students leaving the building how full their class was. However, you soon realize that this sample is not random because you only went to only one of the buildings and the classes at that building may not be representative of all 8:00 AM classes. Moreover, since the students you surveyed were the first to exit the building, it's also quite possible that they all came from the same class! Based on what we have learned, how can we ensure that we choose a sample of students that is representative of all 8:00 AM classes that take place on a given morning