Dr. fletcher is interested in whether joining a fraternity/sorority causes people to become more concerned about their attractiveness. She recruits a group of 55 freshmen (25 men, 30 women) who are planning to go through fraternity/sorority recruitment on her campus. After they join, she gives them a measure of attractiveness concern (the Body Concern Scale). In addition to measuring the group of participants who joined a fraternity/sorority, Dr. Ba decides to give the same measure to another group of 55 participants who decided to not join a fraternity/sorority. Doing this would help Dr. Ba address all the hreats to internal validity except: