You examined the prevalence of anti-drug campaigns at several high schools. You finally select three high schools where the students appear similar in terms of IQ scores, family income, family religion, and the like. You match three classes from each grade (9 to 12) from each high school. In one high school, you plan to offer a course on the detrimental effect of drug usage. In the second high school, you plan to have students follow their "normal" curriculum. In the third high school, you plan to use social media as the tool. After obtaining permission from the principals, parents, and children to do your research, you arrive at the first school to begin the course. You find that the class you had selected is on field trips and that the principal assigned a different class at the same grade level to you. This case illustrates the problem(s) of: