REDUCING CLASS SIZES IS ONE OF TODAY'S MOST popular education reform strategies. The Education Commission of the States estimates that such efforts cost states $2.3 billion during the 1999-00 school year alone. The federal government contributed another $1.6 billion in 2000-01 toward meeting the Clinton administration's goal of decreasing class size nationwide in the early grades to no more than 18 students. During the past year or so, the deteriorating condition of state budgets and the Bush administration's new emphasis on accountability have made class-size reduction less of a priority. Yet it remains popular among parents, teachers, and the teacher unions, which often promote it as an alternative to vouchers.
The motivation for reducing class size is intuitive: with smaller classes, teachers should be able to devote more time to each student, both in the classroom and in giving feedback on homework and tests. The concern is at least threefold, First, reducing class size is remarkably expensive, since it requires hiring more personnel. There may be less costly reforms that are at least as effective as class-size reduction. Second, hiring more teachers may dilute the quality of the workforce, thereby negating any gains among the students of good teachers. Finally, the intuitive relationship between class size and teachers' effectiveness may not actually hold true--teachers may be no more successful with 18 students than with 23.
The most persuasive evidence of the benefits of class-size reduction has come from the Project STAR (Student/Teacher Achievement Ratio) experiment in Tennessee, where students were randomly assigned to classrooms of varying size. Smaller classes appeared to yield substantial gains among kindergartners and possibly 1st graders in the first year of the program--gains that were maintained throughout their school years. However, a large body of research literature on class-size reduction contradicts the findings from Project STAR.
To lend a fresh perspective on this issue, we use data from the Third International Mathematics and Science Study (TIMSS) to compare the effects of class size around the world. While Americans squabble over whether class size should be 18 or 25 students, teachers in Korean schools routinely face classrooms of more than 50 students. These and other differences, such as the quality of a nation's teachers, can be valuable tools in discerning where, if ever, class-size reductions are likely to be beneficial.
Ascertaining the effect of class size is less straightforward than it might appear. The central problem is that students are not assigned to classrooms randomly. For instance, schools often establish small remedial classes for lagging students or small enrichment classes for the so-called gifted and talented. In addition, school systems may direct students into schools with different average class sizes on the basis of their performance.
Parents also may influence their children's class sizes. They may work hard to move their children to schools with smaller classes, where they are likely to receive more attention. Thus variation in class size may be simply the result, rather than the cause, of differences in student achievement. Estimating the true effect of class size on student performance requires a strategy that looks only at variations in class size that are unrelated to students' previous achievement.
In principle, two such strategies are available. The first is to conduct a randomized field trial along the lines of Project STAR in Tennessee. Unfortunately, while it used a powerful research design, the Tennessee study was flawed in its implementation. For one thing, no data were collected on students' performance before they were assigned to their classrooms, making it impossible to know whether the assignment was truly random. In addition, the teachers were aware of their participation in Project STAR, as in almost any true experiment. …