This question pertains to the following study: "A researcher wishes to determine the effects of an outdoor adventure summer camp on the self-concept of inner-city boys. Boys are randomly assigned to the summer camp and to a control situation. The experimental treatment is for three months. A secondary purpose is to see if age (ages 6, 9, and 12) affects the relationship of the camp effects and self-concept." The researcher states that self-concept is represented by scores on the Tennessee Self Concept Scale. This is an example of a(n)