Dr Fletcher Is Interested in Understanding Whether Joining a Fraternity/sorority

Essay

Dr. Fletcher is interested in understanding whether joining a fraternity/sorority causes people to become more concerned about their attractiveness and appearance. He recruits a group of 55 freshmen (25 males, 30 females) who are planning to go through fraternity/sorority recruitment on his campus. After they join, he gives them a measure of attractiveness concern/appearance concern (the Body Concern Scale, where higher scores indicate higher body concerns).
In addition to measuring the body concern of the participants who joined a fraternity/sorority immediately before and immediately after they join, Dr. Fletcher measures them for the 3 weeks before and the 3 weeks after. Based on the results below, explain whether Dr. Fletcher should be concerned about a testing threat to internal validity.