Professor Kim's students designed an experiment to investigate whether self-esteem was affected by how often she called on them when they raised their hand in class. They planned to count the number of times each student was called on when they raised their hand, ask each student how their self-esteem was before and after the experiment, and compare their responses based on how often they were called upon. Their causal hypothesis was, "If the professor calls on you more often when you raise your hand, your self-esteem will go up." What is one significant flaw in their design?