A researcher was interested in determining whether test performance of students could be improved by providing them with individual tutoring. The researcher randomly divided fifty first-year college students into two groups. In Group A the instructor lectured to them for three hours per week. Group B received an equal amount of lecturing, but their instructor also met with each student for thirty minutes every week. Each group took the same final exam. The test performance of students in Group B was about the same as that of students in Group A. This study is an example of a(n)