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 'Surveys indicate that politicians with law degrees are better at what they do than politicians without law degrees. These surveys do not prove that having a law degree makes one a better politician, since it is possible that many