Brittany is developing a sampling plan for a study of alcohol consumption among college students at BigState 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 fraternities and sororities to nonmembers. Finally, she wants to consider patterns by gender. Brittany plans to cross-reference BigState University's student directory with membership lists from each fraternity and sorority on campus to develop a sample frame. At BigStateUniversity, there are more female students than male students. However, because Brittany is particularly interested in male drinking behavior, she plans to include an equal number of men and women in the sample. What type of sampling will Brittany use