Intuitively, it seems obvious that reducing the number of pupils in a class will improve the quality of teaching and learning, for example, by increasing the amount of high quality feedback or one-to-one attention learners receive.
There is no doubt that parents like small class sizes. However, Professor John Hattie in his book, Visible Learning: a synthesis of over 800 meta-analyses relating to achievement says that class size doesn't really matter. Or rather, in terms of the effectiveness of all available interventions, reducing class size has one of the smallest impacts.
Reducing class size is very expensive because it means a school has to recruit more good teachers.
Interestingly, research suggests there are less costly interventions which are actually more effective in raising attainment (Hattie lists these*).
The author and myth-buster Malcolm Gladwell points out in his book David and Goliath: underdogs, misfits and the art of battling giants that 77% of Americans believe it makes more sense to use taxpayers' money to lower class sizes than to raise teachers' salaries.
Yet an evaluation of California's Class Size Reduction Programme found the relationship between reducing class sizes and improved academic achievement 'inconclusive' (Bohrnstedt and Strecher).
Eric Hanushek's research on class size reduction also raises substantial doubts over its impact on student attainment.
Gladwell points out that we have become "obsessed with what is good about small classroom and oblivious to what can also be good about large classes". A classroom containing 18-24 students appears to be the ideal number. Anything less and you lose the unique excitement that comes from a critical mass of engaged students.
The Education Endowment Foundation says of class size ... "overall the evidence does not show particularly large of clear effects, until class size is reduced to under 20 or even below 15." It continues, "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 around 20), then benefits on attainment can be identified."
Surveying studies, Gladwell observes that although really big classes are a problem. there is a happy medium and smaller classes don't necessarily lead to better outcomes. This, he explains, is because teachers don't usually adjust their teaching style to smaller class sizes; instead, they just work less. So the 'disadvantage' of moderately big classes isn't necessarily disadvantageous at all.
Based on this, Gladwell suggests that the relationship between class size and achievement is not linear (such that as class size goes down, learning improves) and is best represented by an inverted U curve. As class size is reduced, learning improves until the optimum class size is reached. However, if class size drops below the optimum, learning declines.
*click here to read our beginners' guide to Professor John Hattie and view a table listing the factors he found affects teaching and learning.