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Rethinking class size: The complex story of impact on teaching and learning

Peter Blatchford, Emeritus Professor in Psychology and Education at University College London

The enduring argument over class size is one of the most contested in education. The argument is between many teachers and practitioners who feel that class size makes a big difference to teaching and learning, and many researchers and policymakers who conclude that the effect is trivial.

In our new book, Rethinking Class Size: The Complex Story of Impact on Teaching and Learning (Blatchford & Russell, 2020), we review the research evidence across different types of research design – correlational, experimental, longitudinal – and show it is overwhelmingly framed in simple terms: how much does class size directly affect pupil attainment (almost always in first language and mathematics)? This is particularly a feature of meta analyses which collate results from different studies to arrive at a summary statistic, usually an ‘effect size’. This describes in seemingly definite terms the degree of association between class size and attainment. The conclusion is almost always that the class size effect is small relative to other more influential educational interventions (see for example, Filges, Sonne‐Schmidt, & Nielsen, 2018; Hattie, 2009; Higgins et al., 2013). For many, this is the beginning and the end of the debate. Such is the seeming lack of importance of class size, some have even argued that class sizes at secondary level can be raised and resources that are saved used for professional development (see Blatchford and Russell, 2020).

Our large-scale research on class size, however – undertaken over more than 20 years, and resulting in many publications – convinces us that the exclusive focus on the association with pupil academic outcomes is a misleading way of addressing the class-size debate, and has led to false conclusions. In Rethinking Class Size we integrate results from these publications, and also include results from new analyses of our detailed data from systematic observations, teacher and other staff interviews and national questionnaires, and detailed case studies of large and small classes. In contrast to a simple class size/attainment link, we show the complex ways in which class size is interconnected with classroom processes. It affects the balance of individual, group and whole-class teaching; classroom management of behaviour; the size, number and management of groups in the class; the quality of peer relationships; the nature of the tasks set in the class; and administrative activities like marking, report writing, planning and preparation. Large classes can present profound problems for teaching, especially where there is wide diversity in pupil attainment levels. It adversely affects the amount and quality of individual support and feedback, the setting up of practical and investigative tasks, and time for marking and preparation. It often leads to more whole-class teaching than teachers would like (Blatchford & Russell, 2020).

‘Our large-scale research on class size … convinces us that the exclusive focus on the association with pupil academic outcomes is a misleading way of addressing the class-size debate, and has led to false conclusions.’

Our research has also shown how large classes can affect teachers themselves, who in a sense soak up the negative consequences, for example by more demanding classroom management, the need for extra individual support, and excessive marking in their own time. This can in turn affect their wellbeing and teacher retention more generally (see Blatchford & Russell, 2020).

There is an additional problem with much of the existing research. Because the focus is just on effects in relation to attainment in first language and mathematics, it tells only part of the story about pupil ‘outcomes’. In Rethinking Class Size, we present evidence that class size also affects other aspects of pupil functioning. Children in larger classes show less on-task and more off-task behaviour, and this is particularly marked for low-attaining pupils (see Blatchford & Russell, 2020, but for more details see also Blatchford, Bassett, & Brown, 2011). Moreover, for many teachers the effects of class size on pupil development are more broadly defined – for example, in terms of creative work, investigative skills and independent working. Unfortunately, these non-academic pupil outcomes have hitherto received very little attention from researchers.

Our conclusion is that results on class-size effects based simply on correlations with academic outcomes are too limited. The effect of class size on pupils is not direct, but bound up in a complex interconnected, dynamic and relational system of classroom processes. In the book we develop a social pedagogical conceptual framework to describe the way this works.

We therefore need to rethink the class-size debate – and rethink research on class size. We do not need any more reviews of associations between class size and academic attainment. We cannot reliably interpret these results without taking account of how teachers and pupils respond to class-size variation. We also need to go further and consider – and research – how teachers can make the most of smaller (and larger) classes. We know, for example, that teachers in small classes do not always change their style of teaching. We need, therefore, dedicated educational research on classroom processes related to class-size differences, and informed pedagogical analysis to inform school practice.

This blog post is based in part on the book Rethinking Class Size: The Complex Story of Impact on Teaching and Learning by Peter Blatchford and Anthony Russell, published by UCL Press on 12 November 2020 on an open access basis. The book can be freely downloaded from the publisher’s website.


Blatchford, P., Bassett, P., & Brown, P. (2011). Examining the effect of class size on classroom engagement and teacher–pupil interaction: Differences in relation to pupil prior attainment and primary vs. secondary schools. Learning and Instruction, 21(6), 715–730.

Blatchford, P., & Russell, A. (2020). Rethinking class size: The complex story of impact on teaching and learning. London: UCL Press. Retrieved from

Filges, T., Sonne‐Schmidt, C. S., & Nielsen, B. C. V. (2018). Small class sizes for improving student achievement in primary and secondary schools: A systematic review. Campbell Systematic Reviews, 14, 1–107.

Hattie, J. 2009. Visible learning: A synthesis of over 800 meta-analyses relating to achievement. London: Routledge.

Higgins, S., Katsipataki, M., Kokotsaki, M., Coleman, R., Major, L. E., & Coe, R. (2013). The Sutton Trust-Education Endowment Foundation teaching and learning toolkit. London: Education Endowment Foundation. Retrieved from