The purpose of the present study is to examine whether teacher effects are moderated by classroom context such as class size. Specifically, we are interested in investigating whether the effects teachers have on student achievement differ considerably by class size (e.g., small or regular size classes). We operationalize teacher effects as classroom-specific residuals that are adjusted for student effects such as gender, race, SES, and school fixed effects. We use the term teacher effects instead of classroom effects to stress the central role of the teacher in the classroom processes. Given that the data were produced from a randomized experiment (i.e., random assignment of students to classes of different size) we assume that class size reduction is the only difference among classes in the baseline and that teachers in different types of classes are the driving force in terms of classroom practices, learning activities, and interactions among students and teachers. In turn, we assume that potential changes in student achievement are due to teachers, other things being equal. 

Over time published research has provided some evidence about classroom practices in smaller and larger classes. Early work has found that students received more attention through questions and probes in small classes than students in other classes, but  teachers in small classes were more likely to teach the class as a whole (Bourke, 1986). Early reviews of the literature (e.g., Odden, 1990) have reported that, in small classes, students are more attentive and teachers have more opportunities for curriculum expansion activities. Another study that used data from second and third grades in Project STAR have found that teachers did not modify their teaching practices considerably in small and regular classes in these grades (Evertson & Randolph, 1989). More recent work has used large scale data to examine how class size change teaching practices and have provided additional evidence that, in small classes, teachers spent more time in individualized instruction and less time on group instruction (Betts & Shkolnik, 1999). In addition, teachers in small classes were more likely to devote time to review activities. The authors concluded that, overall, teachers in small classes did not change their instruction practices dramatically.

Previous work has argued that class size offers the context in which teachers and students interact (Zahorik, 1999). This notion implies that the effects of class size are “mediated” by what teachers and students do in the classrooms. Along the same lines Anderson (2000) points out that, small classes provide opportunities for teachers to modify their instruction accordingly to maximize learning. However, in order to take advantage of the opportunities that small classes provide, teachers need first to comprehend what types of instructional changes need to enacted when they teach small classes (Anderson, 2000). In fact, Anderson provided a useful theory of change model about class size effects that stresses three main factors: in small classes a) there should be fewer discipline problems, b) teachers should have greater knowledge of their students, and c) teacher satisfaction and enthusiasm should be greater.

Anderson, L.W. (2000). Why should reduced class size lead to increased student achievement? In M. C. Wang & J. D. Finn (Eds). How small classes help teachers do their best. (pp. 3-24). Philadelphia: Temple University Center for Research in Human Development and Education. Betts, J. R., & Shkolnik, J. L. (1999). The behavioral effects of variations in class size: the case of math teachers. Educational Evaluation and Policy Analysis, 21, 193-213. Bourke, S. (1986). How smaller is better: Some relationships between class size, teaching practices, and student achievement. American Educational Research Journal, 23, 558-571. Evertson, C. M., & Randolph, C. H. (1989). Teaching practices and class size: a new look at an old issue. Peabody Journal of Education, 67, 85-105. Odden, A. (1990). Class Size and Student Achievement: Research-Based Policy Alternatives. Educational Evaluation and Policy Analysis, 12(2), 213-227. Zahorik, J. A. (1999). Reducing class size leads to individual instruction. Educational Leadership, 57(1), 50-52.