In an effort to improve a statistics course, we want to interview randomly chosen students about their attitudes toward the class. There are 800 students currently enrolled, and six instructors who teach sections of the course. To make sure that students from each instructor are equally represented in the sample, we will first take random samples of size 10 from each of the six classes separately, and then combine these to get the overall sample of students. This type of sample is called a __________ random sample.