A presenter wants to see know if his presentation was effective. In order to do that he feels like he should interview a sample of the participants. When choosing his sample, he believes he should consult approximately 10% of the audience. He doesn't want to just gather the first ones out of the room or the last ones because he is afraid that the time that they leave could bias the results. For example, those in a hurry could have been bored and those that stayed could be more interested. Therefore, to get a more appropriate sample he should use which method