4. An instructor (not in statistics!) claims that after exams he lavishes the best performing students with praise and he admonishes the worst performing students. Then, he claims, the students he praised usually do worse in the next test than in the first exam and the students he admonished do better, proving that punishment is better than praise in motivating students. What concept from the course that best illustrates the flaw in the instructor's thinking? Give a simple probability model for each student's performance in the two tests. (3pt)