EXAMPLE 3.8

Are smaller class sizes better? Do smaller classes in elementary school really benefit students in areas such as scores on standard tests, staying in school, and going on to college? We might do an observational study that compares students who happened to be in smaller classes with those who happened to be in larger classes in their early school years. Small classes are expensive, so they are more common in schools that serve richer communities. Students in small classes tend to also have other advantages: their schools have more resources, their parents are better educated, and so on. Confounding makes it impossible to isolate the effects of small classes.

The Tennessee STAR program was an experiment on the effects of class size. It has been called “one of the most important educational investigations ever carried out.” The subjects were 6385 students who were beginning kindergarten. Each student was assigned to one of three treatments: regular class (22 to 25 students) with one teacher, regular class (22 to 25 students) with a teacher and a full-time teacher’s aide, and small class (13 to 17 students). These treatments are levels of a single factor, the type of class. The students stayed in the same type of class for four years, then all returned to regular classes. In later years, students from the small classes had higher scores on the outcomes, standard tests. The benefits of small classes were greatest for minority students.⁸