Class Size and Student Achievement.

Schooling has multiple purposes. In the long run, higher levels of schooling are associated with higher earnings and economic mobility, better health, lower mortality rates, and greater democratic participation. For these reasons, most societies require children to attend school for a specified number of years or until they reach a certain age. Many of the benefits of schooling occur in part because students learn some new knowledge or skills that enhance their ability to communicate, solve problems, and make decisions. Much of the debate over schooling is essentially about how to maximize the amount of student learning, typically as measured by various assessment instruments such as standardized achievement tests. From a societal viewpoint, since resources—most notably, time—are required for learning, and are scarce, the amount of learning needs to be maximized at least cost. Learning is complex, involving cognitive processes that are not completely understood. Typically, school systems have established a primary mode of learning that involves groups of students of about the same age interacting with a single individual leading activities in a confined physical space, directed toward learning a particular topic—in other words, students are placed in classes. The number of other students in the class can vary. At the extreme, there can be one or more adults facilitating learning—teachers—with one or two students. At the other, a student may be one of a few hundred being taught by a single instructor (or, with new Internet technology, one of millions). The number of students in a class has the potential to affect how much is learned in a number of different ways. For example, it could affect how students interact with each other—the level of social engagement. This may result, for example, in more or less noise and disruptive behavior, which in turn affect the kinds of activities the teacher is able to promote. It could affect how much time the teacher is able to focus on individual students and their specific needs rather than on the group as a whole. Since it is easier to focus on one individual in a smaller group, the smaller the class size, the more likely individual attention can be given, in theory at least. The class size could also affect the teacher’s allocation of time and, hence, effectiveness, in other ways, too—for example, how much material can be covered. Teachers may choose different methods of teaching and assessment when they have smaller classes. For example, they may assign more writing, or provide more feedback on students’ written work, or use open-ended assessments, or encourage more discussions, all activities that may be more feasible with a smaller number of students. Exposure to a particular learning environment may affect learning over the time period of exposure, or it may have longer term or delayed effects (e.g., by increasing self-esteem or cognitive developments that have lasting effects). For these reasons, changes to the class size are considered a potential means of changing how much students learn. Not only is class size potentially one of the key variables in the “production” of learning or knowledge, it is one of the simplest variables for policymakers to manipulate. However, the amount of student learning is dependent on many different factors. Some are related to the classroom and school environment in which the class takes place, but others are related to the student’s own background and motivation and broader community influences. When we ask whether class size matters for achievement, it is essential to ask also, how class size matters. This is important for three reasons. First, if we can observe not only achievement differences, but also the mechanisms through which the differences are produced, this will increase our confidence that the differences are real, and not an artifact of some unmeasured or inadequately controlled condition. Second, the effects of class size may vary in different circumstances, and identifying how class size affects achievement will help us to understand why the effects of class size are variable. Third, the potential benefits of class-size reduction may be greater than what we observe. For example, suppose class-size reductions aid achievement, but only when teachers modify instructional practices to take advantage of the smaller classes. If a few teachers make such modifications, but most do not, then understanding how class size affects achievement in some cases will help reveal its potential effects, even if the potential is generally unrealized.