Your classmate solicits participants who self-identify as having strong test anxiety and then has them watch a video on human causes of global warming before testing them on the material. They then participate in a meditation designed to relax students and have them focus on their strengths as a student. They then watch a video on additional causes of global warming and take a test on that material. She finds that the students score significantly higher on the second test of global warming and concludes that the meditation intervention significantly reduces test anxiety. Identify a flaw in this study and propose a way to address it.