by Paul von Hippel
Started: September 6, 2016. Last updated: September 20, 2016

Definition

Class size reduction is the policy of reducing the number of students who share a classroom–for example reducing the number of kindergarteners per classroom from 25 to 15. The goal of class size reduction is typically to increase test scores and other measures of student achievement.

Reducing class size is not the same as reducing the student-teacher ratio. It is possible to reduce the student-teacher ratio without reducing class sizes if the number of classes per teacher is reduced as well. The relationship between class size and student-teacher ratio looks like this.

• Students/Teacher = Students / Classroom * Classrooms / Teacher

The number of classrooms per teacher may be reduced if a school increases its use of team-teaching, reduces the courseload taught by each teacher, or hires teachers who take administrative, specialist, or support roles rather than taking primary responsibility for a classroom.

Political background: Interest groups and ideologies

Like other education policy debates, the debate over class size reduction is informed by interests and ideologies as well as by evidence.

Some of the strongest proponents for class size reduction have been teacher's unions. Teachers' unions have a clear interest in reducing class sizes. In addition to any educational benefits it might have, class size reduction eases teachers' work conditions and increases the number of teachers that schools must hire. Albert Shanker, who was president of the United Federation of Teachers from 1964 to 1985 and president of the American Federation of Teachers from 1974 to 1997, stated that one of his goals was to reduce class sizes to 10 students (Goldstein, 2015). Helen Bain, who was president of the National Education Association from 1970 to 1971, persistently lobbied the Tennessee Legislature for class size reduction–an effort that eventually led to the Project STAR experiment (discussed later) and the class size reduction policies that followed it (Ritter & Boruch 1999).

Opponents of class size reduction include fiscal conservatives who tend to think that, even if it is effective, a policy which requires hiring many new teachers will be much more expensive than other policies which might have similar effects. One policy maker who shared these concerns was Tennesse legislator Steve Cobb, a key sponsor of Project STAR (Ritter & Boruch 1999). Other opponents include include conservative economists, such as Eric Hanushek (2015) and Carolyn Hoxby (1998), who tend to be skeptical of reforms that simply supply schools with additional resources and hope that improvements will follow. These economists argue that, beyond a certain point, resources reach a point of diminishing returns and reforms must use market principles (incentives, choice) in order to improve student outcomes. Economists also argue that increasing the number of teachers hired and retained makes it difficult to be selective about teacher quality (Hanushek 2015).

Evidence

Effects

The effects of class size reduction have been evaluated many times by research. Most research has estimated effects on test scores, although other outcomes have been looked at on occasion.

As is the case with many education policies, the evidence on the effects of class size reduction is mixed, with some studies suggesting it has large positive effects, and other studies suggesting little or no effect. Scholars have drawn different conclusions, depending on how they weight the evidence.

Scholars who favor class-size reduction tend to put a lot of weight on Project STAR, a randomized experiment conducted in 79 Tennessee elementary schools in 1985-89. Project STAR's results suggested that substantial reductions in K-3 class sizes – from approximately 25 students to approximately 15 – increased test scores by nearly 0.2 standard deviations (Schanzenbach 2006).

Because students and teachers were assigned at random to large and small classes, Project STAR had high internal validity and some scholars give it much more weight than other studies. The economist Alan Krueger (2003), quoting Galileo, has suggested that a single well-designed study is like a "Barbary steed" that "can outrun a hundred dray horses" – where "dray horses" observational studies with weaker designs and less internal validity.

Scholars who are skeptical of class-size reduction tend to put less weight on Project STAR. Notwithstanding its internal validity, Project STAR had limited external validity. Its results are valid for the Tennessee elementary school students who participated 30 years ago, but do not necessarily generalize to other students in other places and times.

Skeptical scholars are especially concerned about the experiences of California and Florida, which have spent billions of dollars on class-size reduction, without experiencing anything close to the effects that were observed in Project STAR (Whitehurst & Chingos). Because they were not randomized, the California and Florida policies cannot be evaluated as rigorously as Project STAR (Schanzenbach 2011), yet they cannot be discounted because they represent the results of large-scale policies whereas Project STAR was just a small-scale pilot (Whitehurst & Chingos).

It is somewhat dispiriting that so much debate revolves around a single experiment that ended almost 30 years ago. There is a neeed for more randomized experiments in education, including a modern replication of Project STAR.

Unintended consequences

Large scale reduction in class sizes can have unintended consequences. The experience of California is the best-known example. To reduce K-3 class sizes from 30 to 20, California had to fill 25,000 new teaching positions in two years. The teachers hired were inexperienced and many were uncertified (Jepsen & Rivkin 2009). When teaching vacancies were created, many senior teachers took the opportunity to transfer to schools serving affluent white and Asian students, so that the new, inexperienced teachers were disproportionately hired by schools serving poor black and Hispanic students (Jepsen & Rivkin 2009). In addition, many schools lacked enough classrooms for the new teachers, and had to use portable classrooms or staggered year-round calendars to accommodate them. The year-round calendars slightly reduced test scores, in part by making it harder to attract and retain experienced teachers in high-poverty schools (von Hippel 2016; and citations therein).

Evaluation suggests that the unintended consequences of class-size reduction approximately canceled out the benefits of smaller classes, at least in the short run. More specifically, reducing class sizes increased test scores by about 0.1 standard deviations (about half the effect observed in Project STAR), but reductions in teacher experience and quality reduced scores by approximately the same amount (Jepsen & Rivkin 2009). Fortunately, some unintended consequences faded as California's new teachers grew more experienced and effective -- but this took several years.

A recent evaluation of Florida, however, suggested that the ineffectiveness of that state's class size reduction policy could not be attributed to a decline in teacher quality. Although teacher quality did decline in Florida, it declined just as much in schools that reduced class sizes as it did in schools that didn't (Dieterle 2015).

Mechanisms

Although most research does not try to explain the effect of class size reduction, some has looked "inside the black box" (Fredriksson et al. 2014) to try to figure out how class size reduction works. A variety of mechanisms have been tested. It has been suggested that class size reduction increases the attention that teachers pay to students, increases student engagement, and reduces student disruption (Fredriksson et al. 2014; and citations therein).

The variety of explanations can be viewed as reassuring in the sense that there are a number of ways in which the reform could have an effect. On the other hand, the variety of explanations also suggests some confusion regarding how the reform actually works – when it does.

References

• Dieterle, S. G. (2015). Class-size reduction policies and the quality of entering teachers. Labour Economics, 36, 35–47. http://doi.org/10.1016/j.labeco.2015.07.005
• Fredriksson, Peter and Ockert, Bjorn and Oosterbeek, Hessel. 2014. Inside the Black Box of Class Size: Mechanisms, Behavioral Responses, and Social Background. IZA Discussion Paper No. 8019. Available at SSRN: http://ssrn.com/abstract=2409541
• Goldstein, Dana. (2015). The Teacher Wars. New York: Anchor.
• Finn, J. D., Pannozzo, G. M., & Achilles, C. M. (2003). The “Why’s” of Class Size: Student Behavior in Small Classes. Review of Educational Research, 73(3), 321–368. http://doi.org/10.3102/00346543073003321 
• Hoxby, C. (1998). The Effects of Class Size on Student Achievement: New Evidence from Population Variation. Quarterly Journal of Economics http://qje.oxfordjournals.org/content/115/4/1239.abstract
• Hanushek, E. (05/23/2011). The Upside of Class Size Reducation. Education Next. http://educationnext.org/the-upside-of-class-size-reduction/ (Note. I believe the title is a typo and is meant to read "The Upside of Class Size Increases.")
• Jepsen, Christopher, and Steven Rivkin. 2009. “Class Size Reduction and Student Achievement: The Potential Tradeoff between Teacher Quality and Class Size.” Journal of Human Resources 44(1):223–250. Retrieved October 14, 2010.
• Krueger, Alan B. 2003. “Economic Considerations and Class Size.” The Economic Journal 113(485):F34–F63.
• Ritter, G. W., & Boruch, R. F. (1999). The Political and Institutional Origins of a Randomized Controlled Trial on Elementary School Class Size: Tennessee’s Project STAR. Educational Evaluation and Policy Analysis, 21(2), 111–125. http://doi.org/10.3102/01623737021002111.