A college administrator knows that 70% of the students at her college are from out of state, so she decides to make sure that she includes 70 out-of-state students and 30 in-state students in her survey about admission practices at the college. She has a list of all the out-of-state and in-state students currently enrolled at the college. She randomly selects 70 students from the out-of-state list and 30 students from the in-state list. Which sampling method is she using?