Jared is conducting an experiment with ESP (extrasensory perception) training. He begins with a pretest of his 40 participants and divides them into two groups based on their scores. The participants with the 20 lowest scores are given extensive training on how to detect the signals. The participants with the 20 highest scores are given no training. Both groups are tested and the average score of the participants with the training improved. Why can't Jared conclude that the training caused the difference