EXAMPLE 3.15 Is the Cost Justified?

The increased costs for teacher salaries and facilities associated with smaller class sizes can be substantial. Are smaller classes really better? We might do an observational study that compares students who happened to be in smaller and 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. The size of the classes is confounded with other characteristics of the students, making 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 with a teacher and a full-time teacher’s aide, and small class (13 to 17 students) with one teacher. 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 standard tests, were less likely to fail a grade, had better high school grades, and so on. The benefits of small classes were greatest for minority students.²⁰