At a college there are 120 students in Year 9, 90 in year 10, 110 in year 11, and 80 in year 12. A school administrator selects a random sample of 12 of the Year 9, a random sample of 9 of the year 10, a random sample of 11 of the Year 11, and a random sample of 8 of the Year 12. She then interviews all the students selected.

Identify the type of sampling used in this example.