Mr. Jacobs wants to find out whether a new program for teaching physical education promotes students' physical development. He gives his students a number of tests before they begin the program (pretests) and the same tests again after they have been in the program for eight months (posttests). He finds that the students' posttest scores are higher than their pretest scores and so concludes that the program is effective. What is definitely wrong with Mr. Jacobs' conclusion?