What is the Impact of Class Size on Student Learning?
by Spyros Konstantopoulos - February 05, 2009
This commentary discusses the impact of small classes on student achievement.
A fundamental objective of American education is to allocate school resources most effectively in order to increase achievement for all students. Many school policies are designed to ensure the best possible distribution of school resources that will result in higher levels of academic achievement for all students. Class size reduction has been identified by some researchers as a mechanism that schools can use to increase student achievement (e.g., Finn & Achilles, 1990). The effects of class size on student achievement have been of great interest to educational researchers and policy makers especially the last two decades. Class size reduction is an appealing school intervention because, first, it is easy to implement, that is, its implementation involves making sure that each class has no more than 20 students per classroom, for example. Second, class size reduction does not necessarily require changes in teaching or instructional practices, or curricula; that is, teachers can go about their everyday school routine.
Reducing class size with the objective of improving student achievement is a policy option that has gained considerable attention nationwide. Many states introduced class-size reduction programs in the 1990s. California, for example, introduced a class size reduction program giving schools financial incentives to reduce class size in the early elementary grades to twenty or fewer students in each classroom. Wisconsin adopted a program that reduced class size to nearly fifteen students per classroom in early grades in schools with high percentages of students of low socioeconomic status.
The effects of class size reduction on student achievement have been examined empirically via various research designs and analyses over the past few decades. Numerous small-scale experimental and quasi-experimental studies have investigated the effects of class size on student achievement. Reviews of these studies have suggested that class-size reduction has positive effects on student achievement and that these effects become greater as class size becomes smaller (Glass, & Smith, 1979; Glass, Cahen, Smith, & Filby, 1982). The benefits of small classes appeared to be more pronounced in early elementary grades, in classes with less than twenty students, and among some minorities and those of low socioeconomic status.
Other studies have examined the effects of class size reduction on student achievement using data from non-experimental studies. Typically, these studies compute the association between class size and student achievement, adjusting for important factors in student background such as gender, race and ethnicity, socioeconomic status, and previous academic achievement. The interpretation of these studies results has been controversial; hence, this body of research has not yielded clear evidence about the effects of small classes. Some reviewers of this research have argued that the effects of class size on student achievement are small or nonexistent (Hanushek, 1989). However, others have suggested that reducing class size has considerable effects and that students benefit from being in small classes (Greenwald, Hedges, & Laine, 1996). Findings from primary studies during the last decade have also been mixed. For instance, Angrist and Lavy (1999) found that reducing class size increased fourth and fifth graders scores significantly, and Pong and Pallas (2001) found positive small class effects on eighth-grade achievement. In contrast, Hoxby (2000) reported that smaller classes had little to no effect on student achievement, and Milesi and Gamoran (2006) found no evidence of class size effects on student achievement.
Evidence from Project STAR
High-quality experimental data emerged from the Tennessee class size experiment or Project STAR (Student Teacher Achievement Ratio). Project STAR is a large-scale experiment widely regarded as one of the greatest experiments in education in America. Nearly 80 schools and more than 11,000 students participated in Project STAR over a four-year period (kindergarten to third grade). Each year, within each school, students and teachers were randomly assigned to small and regular size classes. Early analyses of Project STAR data indicated that small classes had positive effects on student achievement in early grades (Finn & Achilles, 1990). Later, more sophisticated and thorough analyses of the same data also demonstrated that small classes increase student achievement for all students (Krueger, 1999; Nye, Hedges, & Konstantopoulos, 2000a). Hence, the findings from studies that used high-quality experimental data have been consistent and have provided clear answers about the positive immediate effects of small classes on student achievement.
In general, research has shown that the initial positive effects of education interventions tend to fade within a few years. Interventions in the early grades typically dissipate over time by the end of elementary school. Project STAR demonstrated important immediate benefits of small classes for all students. To determine whether these effects dissipated over time students who participated in Project STAR were followed for several years through middle school. Strong evidence suggested that the positive effects of small classes on student achievement persisedt through the eighth grade (Finn, Gerber, Achilles, & Boyd-Zaharias, 2001; Nye, Hedges, & Konstantopoulos, 1999). The findings also indicated that the lasting benefits of small classes became greater the longer students were in small classes in earlier grades. For example, the lasting benefits for students who were in small classes for all four years, kindergarten through third grade, were nearly double the benefits for students who were in small classes for only one year. This suggests that one year of exposure in small classes is not enough, and that students benefit more from additional years in small classes in early grades. It appears that the long-term benefits of small classes are evident through high school grades as well.
Project STAR data have also been used to examine whether small class size benefits low-achieving, minority, and disadvantaged students more than other students. The findings of these studies are rather mixed and inconclusive. Some studies have reported that class-size reduction has greater positive effects for minority students in kindergarten and first grade (Finn & Achilles, 1990). However, other researchers have found only weak evidence that class size reduction had greater benefits for minority or disadvantaged students (Nye, Hedges, & Konstantopoulos, 2000b). In contrast, a recent study that used data from project STAR documented that high achievers appear to benefit more from being in small classes in early grades than low achievers (Konstantopoulos, 2008). That is, although all students seem to benefit from being in small classes, there is no evidence that small classes can close the achievement gap between minority and white or rich and poor students. In fact, there is some evidence that class-size reduction may widen the achievement gap between low and high achievers in early grades.
Analyses using Project STAR data have provided conclusive evidence that class size reduction has important positive immediate and long-term effects on student achievement for all students. Note that, on average in Project STAR, there were 15 students in small classes and 23 students in regular classes. This eight student difference on average between small and regular size classes resulted in increases in student achievement that were nearly one-fifth of a standard deviation or one-fourth of the average annual gain in achievement in early grades (see Hill, Bloom, Black, & Lipsey, in press). The additive effects of small classes from kindergarten through third grade produced achievement gains that were nearly one-third of a standard deviation or less that one-half of the average annual gain in achievement in early grades. There are substantial effects for educational interventions. It is also noteworthy that the long-term benefits of small classes are only slightly diminished over time and are slightly smaller in magnitude than the immediate small class effects in early grades.
However, there is no conclusive evidence that class size reduction can close the achievement gap between minority and white or rich and poor students. Still, because all students benefit from being in small classes minority and disadvantaged students are better off in small classes than in larger classes. Because the achievement gains from Project STAR appear to be substantial and to outweigh the cost of the study (Krueger, 2003), I argue that class size reduction programs have the potential to increase student achievement considerably for all students and be cost effective. That is, class size reduction programs seem a good investment for our education advancement. The successful implementation of class size reduction programs to a large scale (e.g., throughout a state), however, requires sufficient allocation of funds for building new classrooms and hiring new well-qualified teachers. States and school districts need to plan carefully the implementation of class size reduction programs and assure long-term commitment and investment for such programs. Class size reduction programs are more likely to be effective when school organizational and pedagogical conditions are intact.
Angrist, J. D., & Lavy, V. (1999). Using Maimonides rule to estimate the effect of class size on scholastic achievement. Quarterly Journal of Economics, 114, 533-575.
Finn, J. D., & Achilles, C. M. (1990). Answers and questions about class size: A statewide experiment. American Educational Research Journal, 27, 557-577.
Finn, J. D., Gerber, S. B., & Boyd-Zaharias, J. (2001). Small classes in the early grades, academic achievement, and graduating from high school. Journal of Educational Psychology, 97, 214-223.
Glass, G. V., Cahen, L.S., Smith, M. E., & Filby, N. N. (1982). School class size: Research and policy. Beverly Hills, CA: Sage.
Glass, G. V., & Smith, M. E. (1979). Meta-analysis of research on class size and achievement. Educational Evaluation and Policy Analysis, 1, 2-16.
Greenwald, R., Hedges, L. V., & Laine, R. D. (1996). The effects of school Resources on student achievement, Review of Educational Research, 66, 361-396.
Hill, C. J., Bloom, H. S., Black, A. R., & Lipsey, M. W. (in press). Empirical benchmarks for interpreting effect sizes in research. Child Development Perspectives.
Hanushek, E. A. (1989). The impact of differential expenditures on school
performance. Educational Researcher, 18, 45-51.
Hoxby, C. M. (2000). The effects of class size on student achievement: New evidence from
population variation. Quarterly Journal of Economics, 115, 1239-1285.
Konstantopoulos, S. (2008). Do small classes reduce the achievement gap between
low and high achievers? Evidence from Project STAR. Elementary School
Journal, 108, 275-291.
Krueger, A. B. (1999). Experimental estimates of education production functions.
Quarterly Journal of Economics, 114, 497-532.
Krueger, A. B. (2003). Economic considerations and class size. Economic Journal, 113, 34-63.
Nye, B., Hedges, L.V., & Konstantopoulos, S. (1999). The long-term effects of small classes: A five-year follow-up of the Tennessee class size experiment. Educational Evaluation and Policy Analysis, 21, 127-142.
Nye, B., Hedges, L.V., & Konstantopoulos, S. (2000a). Effects of small classes on
academic achievement: The results of the Tennessee class size experiment.
American Educational Research Journal, 37, 123-151.
Nye, B., Hedges, L.V., & Konstantopoulos, S. (200b). Do the disadvantaged benefit from
the small classes? Evidence from the Tennessee class size experiment. American
Journal of Education, 109, 1-26.
Pong, S. L., & Pallas, A. (2001). Class size and eighth-grade achievement in the United
States and abroad. Educational Evaluation and Policy Analysis, 23, 251-273.