Theorists believe that early exposure to structured exercise will increase artistic ability. A group of 15 pairs of identical twins are selected for an experiment to test this theory. One of each pair is randomly assigned to a twice-weekly exercise program for one year. The other twin exercises as much or as little as he or she wishes. After one year, each twin is given the same test of artistic ability (higher scores indicating higher ability). The persons judging the test do not know which half of each pair has participated in the exercise program. The results are as follows: