Do smaller class sizes really improve student outcomes?

While it might feel like having fewer pupils in your class would help you make more progress, there is strangely little hard evidence that reducing class sizes consistently improves student attainment.

It's not just teachers who gravitate towards a leaner classroom; reducing class sizes is extremely appealing to parents and one of the main attractions of the independent sector.

You can see the logic: as there are fewer pupils, the range of approaches a teacher can employ and the amount of attention each student will get increases. It seems obvious that having fewer pupils in a class will improve the quality of teaching and learning. Leaving more time for strong feedback and one-to-one attention from the teacher for example.

When analysing a list of factors that affect student learning however, researcher and academic John Hattie found that class size mattered much less than other agents. Most interventions have some positive effects, but other moves – such as teacher efficacy, parental involvement and even school size – had more of an effect upon learning than the number of pupils in a class.

This is particularly important in our cash-strapped era of education. It is expensive to reduce class sizes; schools have to recruit more teachers and ensure they are well trained before they can have any sort of impact. This is expensive, and Hattie's research suggests there are other more cost efficient – and effective – interventions.

The Pisa international tables also suggest that small class sizes do not necessarily create high-performing education systems. Andreas Schleicher, director of education and skills at the OECD, which runs the Pisa research, says: "Everywhere, teachers, parents and policy-makers favour small classes as the key to better and more personalised education... And yet, Pisa results show no relationship between class size and learning outcomes, neither within nor across countries."

The countries at the top of the Pisa tables spend their money elsewhere says Schleicher, namely, in their professionals. They prioritise spending money on better quality teachers – for example by offering competitive salaries, investing in professional development and managing teacher workload – rather than on making classrooms smaller.

The prevailing view among parents, however, is a little different. In his book David and Goliath, thought leader Malcolm Gladwell points out that 77% of Americans believe it makes more sense to use taxpayers money to lower class sizes than to raise teachers' salaries. In fact, when the governor of California announced plans to reduce the state's class sizes, his popularity doubled within three weeks.

Tennessee's Project STAR (Student-Teacher Achievement Ratio), which was commissioned in 1985, had inspired the governor to act. The state's interest was piqued by research in Indiana which showed reducing class size improved student academic achievement. Mindful of the costs, however, they set up a research project to assess what impact their Class Size Reduction (CSR) programme had. The Tennessee Project STAR showed that smaller classes really benefited students in early education; further studies went on to show that this positive effect remained and that those enrolled in small classes early in their school life continued to outperform their peers in larger classes.

When the governor of California did the same, however, the four-year evaluation of California's class reduction programme ultimately described its impact as "inconclusive".

Gladwell points out that we have become "obsessed with what is good about small classrooms and oblivious to what can also be good about large classes". A classroom containing 18–24 students appears to be the ideal number, he says. This is because teachers don't usually adjust their teaching style to smaller class sizes, and anything less and you lose the unique excitement that comes from a critical mass of engaged students.

When Gladwell talked with teachers about class size he found there was general agreement that large class sizes can impair learning. However, he also discovered that teachers believe the same is true about small class sizes. Teachers agreed that when classes became too small the group dynamics became difficult, and individual students were more easily able to dominate the group and disrupt learning.

Analysis from the Education Endowment Foundation suggests something similar. It found that only when you get to classes under 20, or even 15, are there meaningful effects because it is at this point teachers can change their approach. It says: "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 on behaviour and attitudes. In some studies these benefits persist for a number of years (from early primary school through to at least the end of primary school)."

Based on this, Gladwell suggests that the relationship between class size and achievement is actually not linear, but is best represented by an inverted U curve. As class size is reduced, learning improves until the optimum class size is reached. If class size drops below the optimum, however, learning declines. This optimum will vary according to the makeup of the class and the various learning needs of students.

The optimum class size will vary according to the makeup of the class and the various learning needs of students. The optimal class size for a high needs class will be lower than for a class of well-adjusted, independent students. As Gladwell points out in a note in the book, children with special needs do seem to benefit the most from being in a small class.