Victor wants to see if positively greeting children as they enter the classroom reduces problem behaviors. He randomly assigns 10 second- and third-grade classrooms to one of two conditions: classes where the teacher greets each student by name and with a smile as they enter the classroom and classes where the teacher does not greet students. During a 4-week period, he asks teachers to track the number of problem behaviors per classroom on a daily basis via a strict protocol he has designed. What is the control group