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Expert opinion from IOE, UCL's Faculty of Education and Society


Does class size matter? We’ll get a better answer if we rethink the debate

By Blog Editor, IOE Digital, on 13 November 2020

Peter Blatchford and Anthony Russell.

For many teachers, large classes present problems which adversely affect their practice and their pupils’ learning. This is what our surveys show. But researchers and commentators often have a different view. For them the class size debate can be summed up with the question: does class size affect pupil attainment?

As we show in our new open access book, ‘Rethinking Class Size: The Complex Story of Impact on Teaching and Learning’, published by UCL Press this week, researchers (contrary to a practitioner view) commonly find that the statistical association between class size and attainment is not marked and so conclude that class size does not matter much. This has led some to even suggest that we could raise class sizes, and instead invest savings in professional development for teachers. Currently, in the wake of the Covid pandemic and teacher absences, there are reports of some schools being forced to create supersized classes of 60 pupils.

The view that class size is not important is probably the predominant view among researchers and policy makers, and so they may be relatively relaxed about increases in class size. We therefore need – more than ever – good quality evidence on class size effects, but in our view much research is limited and leads to misleading conclusions.

We identify three problems. (more…)

Class size and teaching: width and quality both matter

By Blog Editor, IOE Digital, on 3 February 2017

Peter Blatchford
David Aaronovitch is a good journalist and there is much to admire in an article he wrote for the Times newspaper last week (‘Teachers must get out of their ideological rut’, January 26, 2017). I suspect however that he has never taught a class of children. The line he takes – following the well publicised view of the head of OECD educational policy Andeas Schleicher – is that class size doesn’t matter. David remembers the words of his grandmother: ’feel the quality not the width’.
With my colleague Tony Russell, I am working my way through the carefully collected views of hundreds of primary school teachers, headteachers, Teaching Assistants and pupils, along with careful classroom observations and case studies (part of a large scale study we conducted at UCL Institute of Education), and what stands out are the many ways that class size does indeed matter. We have found that having fewer children in the class tends to mean more individual attention, a more active role for pupils in class, better relationships between pupils, easier classroom management, more individualisation for (more…)

Moving on from the class size debate: a new project with a practical purpose

By Blog Editor, IOE Digital, on 17 February 2015

Peter Blatchford
The head of the OECD PISA surveys, Andreas Schleicher, has been called the most powerful man in education. On  the BBC website this month he described 7 big myths about top-performing school systems. Myth number 4 in Schleicher’s list is that small classes raise standards. He argues that “everywhere, teachers, parents and policy makers favour small classes as the key to better and more personalised education.” In contrast, he argues, high performing education systems invest in better teachers and high performing countries (many in East Asia) have large classes – so the size of a school class can’t be important.
Far from being a myth, however, my sense is that the view that class size is unimportant is in fact becoming more and more accepted by many (more…)

Should we raise class sizes and reduce the number of Teaching Assistants?

By Blog Editor, IOE Digital, on 30 May 2013

Peter Blatchford

Ask any teacher about whether class size matters and the chances are they will say that of course, a smaller class allows for better teaching and learning. Ask any teacher whether having a teaching assistant (TA) in the classroom is beneficial and it’s highly likely they will say they are a great help.

These points seem common sense and certainly correspond to the findings of the IOE’s large scale surveys of teachers’ views. Yet, recently the Think Tank ‘Reform’ has argued that class size and TAs are not important and in the case of the former can be raised without harm and in the case of the latter can be reduced in number. This conclusion is similar to that in several high profile and widely cited reports from OECD, McKinsey, Gratton Institute and Brookings. How do we reconcile these two different perspectives – the practitioner and policy perspectives? Are teachers wrong, as some commentators imply?

To address this question we need to examine the evidence on the relationships between class size and pupil performance, and TAs and pupil performance.

To take class size first: it is striking how much recent reports base their conclusions on three sources of data: cross country comparisons, meta analyses and econometric analysis. All of these, I believe, only offer a partial view and are therefore flawed as evidence of a causal role for class size.

Results from international assessments such as the Program of International Student Assessment (PISA) show that students in the East Asia do very well and also have relatively large classes, and it is often therefore concluded that class size is not important to academic achievement. But the flaw in this argument is not considering the reasons why high performing education systems in places like Hong Kong do well, including high levels of parental support, cultural factors that favour education and the prevalence of private tutoring.

Meta analyses are based on a large scale statistical analysis of multiple studies related to the effects of class size. The conclusions have generally been that class size does not have a large influence on student learning. Yet often these analyses include studies of varying quality, age of pupil, research design, etc.

The same problem exists with econometric analyses – studies by economists who often take measures of class size, or more usually pupil teacher ratios, and develop statistical models of effects that take little account of what actually happens in school.

Interestingly these sources of data are also all secondary analyses, that is, they typically use data collected by other people. In contrast, evidence from two dedicated studies of class size – the Student/Teacher Achievement Ratio (STAR) experimental project from Tennessee and the Class Size and Pupil Adult Ratio (CSPAR) study, which I directed here at the IOE, arrive at similar conclusions: class size does matter for the younger pupils in school and small classes are particularly beneficial for the more disadvantaged and initially lower attaining pupils.

A limitation of the research examined so far is that it is all restricted to the relationships between class size and academic performance. There has been less research on class size and other aspects of classroom life, such as teaching approaches, teacher-pupil interactions and pupil behaviour. The research we have suggests that class size does affect the amount of individual attention pupils receive, and their engagement and active involvement in class. At the IOE we found in a recent study that lower attaining secondary pupils were more likely to be off task in larger classes, when compared to middle and high attaining pupils[1] Intriguingly, in several countries in Asia, including Hong Kong and Shanghai, class size reduction initiatives have been introduced – not so much to affect educational attainment (after all these countries perform very well in the PISA surveys) but to help teachers bring about higher order thinking and collaborative learning experiences.

And for me this helps explain the disparity between the views of teachers and policy commentators: when thinking about the effects of class size, teachers have in mind a broader and less easily measured set of qualities than simply scores on an achievement test. If teaching were simply about delivering a lecture then ‘Reform’ and others like them would have a strong case. But as all teachers know, teaching is not simply about presenting information.

And what about TAs? Interestingly, the research evidence on which most people draw seems to be the IOE’s Deployment and Impact of Support Staff (DISS) project in which we found convincing evidence that pupils with more support from a TA made less progress at school compared to similar children who has less support. It is therefore quite understandable that some have concluded that investment in TAs, e.g. through the Pupil Premium, is not a good use of money. But what’s important here, is how one interprets the results; in a recent book[2] my colleagues and I argue that the most likely explanation for these results is the way that TAs are used in schools and prepared for their work. Put simply: TAs are not used to the best advantage. Often TAs, with little preparation or training, are assigned a one-to-one remedial role with low attaining pupils or those with special educational needs (SEN). We suggest that this is misguided and helps explain the negative impact on these pupils. Also in another recent book we show, on the basis of a two year long action research project, how schools can fundamentally rethink the way they use TAs, so that they add value to teachers rather than replace them in the case of the most disadvantaged pupils[3].

One of the problems with the debates over class size and TAs is the way that it is presented as a binary choice: either invest in class sizes or in teaching. But the point is these are not mutually exclusive. I suspect we all agree that the quality of teaching is vital, but smaller classes and TAs can help teachers provide a more effective education for pupils. This will not happen automatically, but requires careful attention in schools in order to make the most of the opportunities that smaller classes and TAs offer.

So my conclusion is clear: raising class sizes and reducing the number of TAs are very bad ideas!

Peter Blatchford is Professor of Psychology and Education at the Institute of Education, University of London.

For more on the class size topic see: Blatchford, P. (2012) Class size: is small better? In Adey, P and Dillon, J. (Eds) Bad Education: Debunking Myths in Education. Open University Press: Maidenhead, UK –


For more on Teaching Assistants see: www.schoolsupportstaff.net


1 Blatchford, P., Bassett, P., and Brown, P. (2011) Examining the effect of class size on classroom engagement and teacher-pupil interaction: differences in relation to prior pupil attainment and primary vs. secondary schools, Learning and Instruction, 21, 715-730


2 Blatchford, P., Russell, A., and Webster, R. (2012) Reassessing the Impact of Teaching Assistants: How Research Challenges Practice and Policy. Abingdon, Oxon, UK: Routledge


3 Russell, A., Webster, R. and Blatchford, P. (in press, 2013) Maximising the impact of teaching assistants: guidance for school leaders and teachers. Routledge 










Class size or teaching quality? The pendulum swings again

By Blog Editor, IOE Digital, on 27 April 2012

Peter Blatchford
The class size debate has opened up once again. The prevalent view this time seems to be that class size is unimportant. This was apparent in the media coverage of one London council leader who wanted the cap of 30 in a class for the youngest pupils to be relaxed. The thinking is that reducing class sizes is not a cost effective use of public funds and that money could be better spent, particularly on improving teaching quality.
More worryingly, more and more people are now arguing that class sizes could even rise. The judgment of teachers themselves – which we have found is overwhelmingly that teaching and learning are better in smaller classes – often gets overlooked or even denigrated as self serving.
For me, much of the commentary on this topic contains a common but flawed way of thinking about class size. No one could disagree that teachers and teaching quality are the most important ingredient in successful education. But the two biggest studies of class size effects – the STAR project from Tennessee and our own study based at the IOE – are also clear: the number of pupils in the class significantly affects young pupils’ educational progress. The mistake I think is to assume a choice has to be made, as if teaching and class size are mutually exclusive. The number of pupils in a class is an important contextual influence on the quality of teaching which takes place, and the important thing is the way that class size and teaching interconnect.
The question I think needs to get asked more is what teaching approaches work best in smaller (or larger) classes? This is a particularly pressing question in a number of countries in Asia. In Hong Kong for example, despite high performance in international comparisons of academic success, they are reducing class sizes as a matter of policy. In our extensive series of observation studies of class size effects, as part of the IOE research, we found that in smaller classes there was more individual attention and more pupil classroom engagement. But other research has also found that teachers do not always alter their style of teaching when faced with fewer pupils in the class, and could therefore benefit from help with effective strategies in small classes.
During recent visits to primary schools for a current research project on the educational supports given to pupils with special educational needs (SEN) in mainstream classes, I have been struck by one way that class size and teaching are intimately related. It is clear that teachers face an extremely difficult task when seeking to teach pupils in often large and very varied classes that sometimes include pupils with special educational needs.  
Those who argue that class size effects are unimportant seem fixated on evidence exclusively couched in terms of academic achievement test scores. But there are other features, including skills at differentiating the curriculum for pupils at very different levels, which are affected by class size but rarely get a mention, and still less studied in research.
So pitching class size against teaching is far too simplistic. They are both important. A key task for the future is to develop and evaluate effective teaching approaches when faced with different numbers of pupils in the class.