Dr. Johnson conducted a research study in which he investigated the effect of three different methods of teaching mathematics to three different classes of fifth grade students. However, one of the classes had smarter students than the other two and as a result this class demonstrated the most improvement in their mathematics scores. In spite of this problem, he concluded that the method used to teach mathematics to this class of students was the most effective. In this experiment, the intellectual ability of the students in the various classes was: