A psychology researcher investigated the impact of 'positive self-talk' on the athletic performance of college athletes. For the study, a sample of n = 40 athletes were recruited from a state college. Twenty students engaged in positive self-talk for 30 minutes a day for the entire semester; the remaining 20 students did not engage in positive self-talk. At the end of the semester the athletic performance of the athletes was compared. The 40 students are an example of: