Jared is conducting an experiment with extrasensory perception (ESP) training. He begins with a pretest of his 40 participants and then divides them into two groups based on their scores. The participants with the 10 lowest scores are given extensive training on how to detect the signals. The participants with the 30 highest scores are given no training. Both groups are retested and the average score of the participants with the training improved, whereas the average score of the participants without the training fell. Which threat to internal validity should Jared consider?