Researcher: Results indicate that the higher their educational level, the better are students' mathematical skills. These results do not prove that education improves mathematical skills, however, since it is possible that students who have better mathematical skills to start with are the students who reach higher educational levels.
 The reasoning of the researcher's argument is most similar to that of the argument that results from competition indicate that professional athletes outperform amateur athletes but these results do not prove that becoming a professional athlete improves one's athletic performance, since it is possible that the athletes who become professionals are those whose performance is better