What`s the evidence on ……….Class size?

Parents and teachers tend to favour smaller class sizes, but what does the evidence say?

The Education Media Centre is building a series of articles setting out the research evidence on a range of education issues which interest journalists and matter to the public.

Here Robert Coe, Professor in the School of Education at Durham University, looks at the research into the impact of reducing class size. Any opinions expressed are his own.



What difference does the number of pupils in a class make to their learning? The research evidence might surprise you. You may think it incredible and want to reject it as irrelevant and wrong. Your gut instinct might tell you fewer pupils automatically mean significant improvements in learning. That seems obviously true, but the evidence says not. Reducing class sizes does make a difference, but not that much.


  • Reducing class sizes can improve learning a little, but other changes, for example to the quality of teaching, make a bigger difference.
  • Even large reductions in class size, say from 30 to 15, have only a relatively small impact on attainment.
  • The quality of teaching has a greater impact than altering class size. Learning and attainment are likely to be better in a large class where there is high quality teaching than in a small class with mediocre teaching.
  • The high cost of employing extra teachers to reduce class sizes, coupled with the relatively small effect on learning, means lowering class size is unlikely to be the most cost effective way of improving attainment.


Investigating the impact of reducing class size is one of the most researched areas in education, and one where the evidence is both clearest and most consistent . Some studies, including most of the UK-based evidence, compare outcomes for children taught in different sized classes, after taking account of other influences such as pupils’ prior attainment and their home background. Other studies deliberately allocate teachers and children at random to different classes. One of the best of the latter (‘Project STAR’) allocated every child and teacher in the US state of Tennessee to either a large or small class over the first three years of their schooling. Each type of study has different strengths and limitations, but the results broadly agree: reducing class size has a small effect.


In the UK, the leading academic in this field is Professor Peter Blatchford at the Institute of Education in London (see http://www.classsizeresearch.org.uk/). The best summary of all the international research evidence and its implications is a 2005 paper by John Hattie in the International Journal of Educational Research (http://dx.doi.org/10.1016/j.ijer.2006.07.002). The Sutton Trust-EEF Toolkit provides an accessible summary (http://educationendowmentfoundation.org.uk/toolkit/approaches/reducing-class-size )

In the late 1980s I was teaching in a sixth form college.Then funding for A level teaching was generous by today’s standards and a typical group was 8-10 students. If it rose above 15 we would split it in two – unheard of now! In time funding was squeezed and class sizes rose typically to the low twenties. We complained, reduced marking and found it harder to keep up. But it was never obvious that student achievement changed much.

So when teachers and parents say, ‘In smaller classes you can give more individual attention, more feedback, target specific misconceptions, and give personal encouragement. Doesn’t that help learning?’ I think the answer is, ‘Yes, but not much’.

For example, take a class of 30 and spend an hour`s lesson giving individual attention, feedback and support, each student gets two minutes. With a class of 15, that doubles to four minutes – still a small proportion of the total lesson. Compare how much individual attention a pupil gets in a small and larger class, and the overall difference may not be that big.

However it is likely a skilful teacher can produce nearly as much learning using explanation, discussion and questioning with the whole class, as they can when giving individual attention to a single learner. Most of the difficulties an individual needs help with will be shared.

In that case, giving a lot of individual attention begins to look like quite an inefficient strategy, even with a group of 15. In general, most of the feasible and effective methods a teacher can use to promote learning with a group of 15 will work almost as well with a group of 30. If the choice is between having a good teacher in a large class and a poor teacher with a small class, I would choose the large class every time.

There is a further reason for being wary of reducing class sizes – it’s expensive and arguably poor value for money.

A rough calculation suggests that asking every teacher to teach classes of 22 on a full time-table would cost about the same as having classes of 30 but funding every full-time teacher to be released for a day a week for high-quality professional development.

If that training really was good, the impact on that teacher’s effectiveness would be many times greater than the advantage of the smaller class.  And of course, once a teacher has learned those skills, the benefits are felt each year by every student they teach for the rest of their career. Funding for smaller classes is a continual ongoing cost.

Today I am an education researcher and helped produce an accessible review of the relative cost and benefit of different educational strategies for teachers, commissioned by the Sutton Trust and supported by the Education Endowment Foundation: the Teaching and Learning Toolkit.

The toolkit sets out the research message on the limited effect of reducing class size, and it`s the finding which usually gets the strongest response of disbelief from teachers. But why are they so sure children learn more in smaller classes? Typically they say, “Experience!” But experience is notoriously untrustworthy. Is it a like-for-like comparison? How exactly is this better learning revealed? Are we counting every case, or subconsciously picking those confirming our view? A higher standard of proof is needed, and that`s what the research studies bring.

Evidence on the impact of class size is a good example of research telling us something that jars with intuition. If we reject the evidence we can carry on as we are. But if we try to reconcile the two it can enhance our understanding of teaching and learning, and lead us to new thinking about how we can improve both.


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