Dr. Tanaka is an educational psychologist interested in students’ attitudes toward science and the effect of those attitudes on performance on standardized tests. He chooses his local school district to study. There are 15 high schools, and he randomly chooses five. Then, of the 2,500 students in each of those five schools, he randomly recruits 250 students. This is an example of which of the following sampling techniques?
1. Snowball sample
2. systemaic sample
3. multistage sample
4. cluster sample