It may be that students choose friends of similar ability. We would have to measure the ability of both the student and his best friends. All that would get added to the design, plus some way to analyze the ability data to rule out spuriousness. Those techniques are beyond the scope of this course, but you should acknowledge that they need to be incorporated into the design. Assuring Generality .
Science is interested in general laws.
A process of induction is used to argue for the widespread applicability of the law. This is called today generalization instead of induction. In the physical sciences materials used in an experiment are generally representative of the class of objects from which they are taken. A steel ball rolling down an inclined plane is representative of any object under the influence of gravity. But when we apply laws to human social behavior we are generalizing from individuals to groups, and we have to be sure that the indivduals in our sample are representative of the group we have in mind, and not some other.