Why does class size affect educational outcomes?

The number in a class can profoundly affect educational outcomes – but more research is needed, says US academic
1st November 2019, 12:04am
Why Class Size Matters
Chris Parr


Why does class size affect educational outcomes?


A number of educational debates appear immune to any attempt at putting them to rest. Child-led versus direct instruction is one; behaviour approaches is another. But just as persistent and emotive is the question of class size. And, according to Larry Hedges, that is despite very good evidence that class size does, indeed, matter.

Hedges is chair of the department of statistics at Northwestern University in Illinois, and a professor of education and social policy, psychology and medical social science. In 2018, he won the prestigious Yidan Prize for his groundbreaking work developing statistical methods for meta-analysis: research where multiple papers are assessed to look for common findings.

One of the areas that he has studied in detail is the relationship between pupil-to- staff ratios and educational outcomes.

He became interested in the topic in the 1980s. There was little academic consensus about the impact of reducing class sizes, despite a 1979 meta-analysis suggesting it did have an impact. The 1979 study by Gene Glass and Mary Lee Smith showed that the effects of reducing class sizes from about 40 to about 30 pupils were not very significant, but as numbers got smaller, right down to three or four pupils, achievement increased more significantly.

"This meta-analysis seemed to show a pattern that made a lot of sense to many educators," Hedges says, "but that analysis was criticised from a variety of different points of view.

"People thought many of the studies that Smith and Glass included weren't particularly rigorous, and that they were much too lenient in their criteria for evaluating studies, and so it remained a bit controversial."

He adds that economists in the 1980s were "absolutely convinced that the amount of money allocated to schooling … really didn't matter, that it didn't change the achievement of schools to allocate more money to them".

Hedges soon tackled that argument by conducting an analysis of the literature that the economists were using, and he found that, "properly synthesised, they didn't show what the economists thought they showed".

Does class size matter?

Nevertheless, the debates still rumbled on until, in the mid 1990s, a huge experiment was conducted across the entire state of Tennessee to try to ascertain the extent to which smaller classes led to better learning. The results of that study have been a focus of Hedges' work since and he believes it has proved, without doubt, that class size matters.

Officially called the Student/Teacher Achievement Ratio study, it was a four-year longitudinal study funded by the Tennessee General Assembly and conducted by the US State Department of Education. More than 7,000 students in 79 primary schools were randomly assigned into one of three classes: small class (13 to 17 students per teacher); regular class (22 to 25 students per teacher); and regular-with-aide class (22 to 25 students with a full-time teacher's aide).

"The Tennessee legislature wanted to know whether or not they should allocate more money to reduce class sizes," Hedges explains. "They were aware of Glass and Smith's meta-analysis and some other work that suggested class size reductions was a way to improve schooling outcomes, but there was a debate about whether Tennessee ought to put more money into education with the express purposes of reducing class sizes."

Every school in Tennessee was invited to participate in the experiment, and those that did were given enough extra funding to reduce the size of all their classes.

"It was intrusive … They had to agree to let the researchers come in and randomly assign the students in classes, and they had to agree to maintain the class size assignments for four years," Hedges explains.

The study, although imperfect - as some students did move between classes - became one of the most comprehensive investigations into the effects of class size ever conducted.

The experiment "showed pretty definitively that reducing class size increases student achievement", Hedges says.

"Now, the effects aren't enormous, and there are a lot of caveats attached to it … but our analysis showed that there was an immediate effect of reducing class size on achievement, and that the effect persisted through on into high school.

"And others did more follow-up studies of those same students, not just into high school but into the labour market.

"And it turns out that students who were assigned to small classes in kindergarten [Year 1] to third grade [Year 4] actually have better labour market outcomes, they make more money once they're in the labour market, and that is a result that is fairly recent but pretty impressive."

Hedges is keen, though, to make clear precisely what the experiment does and does not show. "What we're pretty sure we know from that experiment is that if you reduce class size from 25 to about 15, that seems to have noticeable effects that last into the labour market," he explains. "But we know less about what would happen if they had reduced the class size to 10, or if they reduced the class size from 25 to 20. We don't know if the effects would be as large, or larger, or if there'd be effects at all. But it's plausible that there would be."

He also points out that the Tennessee class size experiment focused on the early school years - kindergarten to third grade. "We don't know for sure what would happen if you had reduced class sizes later in school," he says.

Lasting effects

Alas, the answer to these questions may be some way away, he adds, given the expense and complexity of carrying out such a large-scale piece of research.

"I think it's probably unlikely that we'll ever get another experiment of this scale on class size," Hedges says.

Despite the positive findings we do have, though, Hedges is of the opinion that class size remains "a very blunt variable" - just reducing class size, he feels, is not the intervention we should take away from this study. Rather, he says, it is more important in the long run to try to understand "what it is that smaller classes enable teachers to do more effectively".

This concurs with the findings of the Education Endowment Foundation's teaching and learning toolkit. It states that "the key issue appears to be whether the reduction is large enough to permit the teacher to change their teaching approach when working with a smaller class and whether, as a result, the pupils change their learning behaviours. If no change occurs then, perhaps unsurprisingly, learning is unlikely to improve.

"When a change in teaching approach does accompany a class size reduction (which appears hard to achieve until classes are smaller than about 20), then benefits on attainment can be identified in addition to improvements in behaviour and attitudes.

"There is some evidence that reducing class sizes is more likely to be effective when accompanied by professional development for teachers focusing on teaching skills and approaches," the guide says.

Hedges believes answering the "why", not the "what", has to be the focus of class size research going forwards, rather than just treading over the same "does it work?" ground. "We need more research on what it is that teachers do, because I believe that curriculum - what you're trying to teach - and instruction - how you're teaching - is ultimately what causes academic achievement," he says.

Hedges believes we owe it to teachers to do this research, as it will mean their hard work can have the biggest impact.

"Just having a smaller class in and of itself is not something that's going to magically improve academic achievement," he says. "But what it will do is enable them to be more effective teachers. And I hope that more research in the future focuses on the kinds of instructional strategies that allow teachers to be more effective."

Chris Parr is a freelance journalist

This article originally appeared in the 1 November 2019 issue under the headline "Tes focus on...class size"

You’ve reached your limit of free articles this month

Register for free to read more

You can read two more articles on Tes for free this month if you register using the button below.

Alternatively, check if your school has a Tes subscription. If not, for just £5 per month you can subscribe personally for:

  • Unlimited access to all Tes magazine content
  • Exclusive subscriber-only articles 
  • Email newsletters

Already registered? Log in

You’ve reached your limit of free articles this month

Subscribe to read more

Check if your school has a Tes subscription. If not, for just £5 per month you can subscribe personally for:

  • Unlimited access to all Tes magazine content
  • Exclusive subscriber-only articles 
  • Email newsletters

Read more