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A 2018 review of the research evidence for reducing class sizes found only small benefits in reading and no benefits in math, on average. Tennessee’s small-class experiment produced strong academic gains in the 1980s but that success has rarely been replicated. Credit: Camilla Forte for The Hechinger Report

Cutting class size appears to be increasingly popular as school districts figure out how to spend their $190 billion in federal money for coronavirus relief, according to media reports. Georgetown University professor Marguerite Roza tracks school spending and she’s also seeing a new 2021 trend of schools’ hiring more teachers in order to reduce class sizes.

Parents and teachers may like smaller classes but the research evidence for spending money on them isn’t strong. Here’s a quick tour through more than five decades of muddy studies.

Until the 1980s, no one had designed a good experiment to see what happens to student achievement when you reduce class size. Earlier research on the relationship between class size and achievement was inconclusive. Sometimes students in small classrooms scored higher than students in large classrooms and sometimes they didn’t

Beginning in 1985, small classes were put to the test in Tennessee where thousands of children in kindergarten through third grade were randomly assigned to small classrooms of about 15 students. Students in the small classrooms gained the equivalent of four extra months of education over the course of three years compared with their peers who learned in larger classrooms of about 22 students. Students were tracked for years afterward and those in small classes continued to outperform. Low-income and Black students reaped extra benefits, narrowing the achievement gap. 

The Tennessee “STAR” (an acronym for Student-Teacher Achievement Ratio) experiment became one of the most widely cited studies in education research. Its success was replicated in Wisconsin in the 1990s. But large statewide efforts in California and Florida failed.

In California, more than $1 billion a year was spent in the mid-1990s to reduce class sizes from 30 to 20 students. Schools needed to create more classes, for example, six classes of 20 students instead of four classes of 30. More than 25,000 new teachers were hired.  

The new job openings attracted veteran educators. Many switched from high-poverty to wealthier schools. High-poverty schools became packed with inexperienced novices. Lower teaching quality offset gains that might have been achieved from the smaller classes. In theory, class-size reduction might have worked if there could have been an excellent teacher for every classroom. In practice, there aren’t enough good teachers to spread around to every student. While Tennessee’s experiment pointed to extra advantages for low-income students, California’s experience suggested that low-income students can be harmed. 

In the research, it’s unclear how low you have to go to trigger the benefits of small classes. Perhaps California didn’t go low enough with 20 students per class. The Tennessee experiment lowered class size to 15 to generate its results. One early childhood study found stronger benefits below eight students, weaker benefits between eight and 15 students, but no benefits above 15 students. One high school study of small science classes concluded that class size had to fall to 10 or fewer students to reap academic gains. That’s hardly practical. 

Tiny benefits from expensive class size reduction efforts are common. Even when some classrooms in Minnesota fell by 10 students, a large reduction, researchers discovered almost minuscule gains in reading in math. “Class size reductions are unlikely to lead to sizable increases in student learning,” the authors of that 2012 study wrote.

Observational studies often find that students appear more engaged in smaller classrooms. But education experts have pointed out that students in large classrooms can be just as engaged with a good teacher who breaks the class down into small groups for hands-on activities, discussions and projects. Changing how teachers teach can sometimes be more powerful than how many students are in the classroom.

Internationally, small classes often don’t make a difference. A large 2017 study of 14 nations in Europe, including Germany, Austria and Italy, found that reducing class sizes neither increased achievement nor reduced achievement gaps. 

A 2018 review of all the high-quality class size experiments around the world, including the U.S., found at most small benefits to small classes when it comes to reading. In math, the review found no benefits at all. “Class size reduction is costly,” the Danish researchers wrote. “The available evidence points to no or only very small effect sizes of small classes in comparison to larger classes.”

Lead researcher Trine Filges, an economist at the Danish Center for Social Science Research, sees the oft-cited Tennessee experiment not as indicative of what class size reduction can achieve but as one “very old study,” now dating back more than 30 years. “One experiment can never stand alone for conclusions to be drawn,” said Filges, by email.

This wouldn’t be the first time that the federal government has spent gobs of money on smaller classes. Back in 1998, Congress enacted the multi-billion-dollar Class-Size Reduction Program with the goal of raising achievement. (Spoiler alert: achievement didn’t soar.) And it’s not news that the research evidence for smaller classes is weak. The Hechinger Report has been covering the inconsistent research evidence for small classes since our newsroom started more than a decade ago. 

The idea of small classrooms has such strong emotional appeal to both parents and teachers that it continually resurfaces. I get it. As a parent, I would want a smaller class for my own child, thinking that she’ll get more attention. When I taught, I felt I could give students more individual help during class and form stronger relationships with students when there weren’t as many of them. No wonder New York City mayoral candidates are currently promising smaller class sizes. That’s an easy way to appeal to both families and the teachers union for votes. 

Small classes are getting a big boost in interest now with billions of dollars flowing to schools and few constraints on how to spend it. Some parents got a taste of super small classes of fewer than 10 students during the pandemic, when it was a necessity to maintain social distance. No one wants to give up that intimacy. 

But if our societal goal is to help students catch up from the months of missed instruction, small class sizes are unlikely to be the right mechanism. Even if we could hire a new cadre of teachers to lead a larger number of smaller classrooms and train them well to become excellent teachers, the coronavirus relief money would be exhausted by the time the novice teachers had finished learning the ropes. We’d have to fire them just as the teachers were approaching their prime. The coronavirus money is a short-term injection and it should be used for short-term solutions. Follow the evidence. Tutoring is a good research-proven place to start. 

This story about class-size reduction was written by Jill Barshay and produced by The Hechinger Report, a nonprofit, independent news organization focused on inequality and innovation in education. Sign up for the Hechinger newsletter.

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  1. I know researchers would like clear cut data and correlation values at r=0.99 but public education is just that, teaching the public. As a teacher you have a finite amount of time with each student. Outside of that time, your student is influenced and affected by their environment. Saying that class size doesn’t matter due to inconclusive test scores is blatantly disregarding what teachers are for students. Smaller classes (15-24) allow for teachers to connect with students, to get to know them personally. I have been teaching science for 25 years and been in classes ranging from 34 Chemistry students to 8 special ed science students. The number of needy students are what make the difference. Even though the 34 chemistry students are were very capable, I was not able to connect with all of them so that when the topics got tougher, I lost some of them. I connected with all 8 special ed students and knew their good days from bad, who needed a granola bar because they missed breakfast, who didn’t have rain gear, and this has nothing to do with test scores. Personal connections help keep students in school, get students through graduation, prevent suicides, get intervention for substance abuse or abusive situations. It is statistically challenging to try and measure this effect, but it is where you need to go to look at the value of small class sizes. Give me two classes that are the same in number, and that will be the only thing in common. One class will have more boys than girls, single parent homes, social-economic differences, ACE scores, previous education experiences, learning disabilities. Class size is very important in education but it is not the singular indicator people want it to be. It is informal, but ask your student who their favorite teacher is and measure their growth in that class, then you will see how important the personal connection is for education.

  2. Jill Barshay ‘s essay regarding the effects of class size on student progress reflects the never ending question regarding class size. Task a parent teaching a child how to bake a cake. Would the parent ask to have fifty children to teach or just one child? Would the experience be better for the teacher and learner in a group of 50 or one to one. Class size matters when the teaching process is altered to meet the needs of one rather than the many. Teaching methodology that reflects the reduction of class size will ultimately help students achieve, Research is often flawed as the dynamics of the classroom and interaction between students and teacher are often overlooked. These variables are crucial to learning and to ignore them is. akin to ignoring learning principles.

  3. I’d love to see some new research on small class sizes that analyze other potential outcomes rather than student academic achievement. In particular, I’m really curious about the potential benefits small class sizes have on student social-emotional well-being and mental health, as well as rates of teacher burnout amongst teachers given a smaller class size and compared to those with larger. Considering the huge increase in mental health challenges as a result of the pandemic and the increasing teacher shortage in many areas of the US, smaller class sizes could be a huge benefit if they have any positive correlation in these areas.

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