In the physical education department, Dr. Shanz asks two of his strength training classes to participate in an experiment. One class will follow a traditional program of index finger strength enhancement, while the other class will try a new, experimental procedure. At the beginning of the semester, the class that will use the experimental method starts out with, on average, weaker index fingers than the other class. At the end of the semester, the class using the traditional method shows no significant increase in finger strength, while the other experimental class now has fingers that are, on average, significantly stronger than those of the control class. This is an example of a(n) effect.