Brittany is developing a sampling plan for a study of alcohol consumption among college students at Metro University. She wants to ensure that students from each class level are represented. She also wants to be able to compare students who are members of Greek organizations to non-Greeks. Finally, she wants to consider patterns by gender. Brittany plans to cross-reference Metro University's student directory with membership lists from each Greek organization on campus to develop a sample frame. In the alcohol consumption study at Metro University, students who do not belong to Greek organizations outnumber those who do by 6 to 4. If Brittany draws the study sample such that 60% of the sample is made up of non-Greek-affiliated students and 40% of Greek-affiliated students, what type of sampling has she used?