Class size and student outcomes in Europe : executive summary in English.
Date: 2018-02-15
Institutions: Directorate-General for Education, Youth, Sport and Culture
Subjects: education policy, organisation of teaching, report, school results, social inequality, socially disadvantaged class, student

Executive Summary in English

European Expert Network on

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33

Edwin Leuven and Hessel Oosterbeek 
January 2018

EENEE Analytical Report No. 33
Prepared for the European Commission

Class size and student outcomes in 
Europe 

Edwin Leuven and Hessel Oosterbeek 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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Luxembourg: Publications Office of the European Union, 2018 

ISBN: 978-92-79-70095-8 
doi: 10.2766/59709 

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This document has been prepared for the European Commission. However, it reflects the views only 
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the information contained therein.

 
 
 
 
 
 
 
 
Executive Summary (English) 

Class size and student outcomes in Europe 

Class  size  reductions  are  popular  with  parents,  teachers  and  politicians.  In  recent  years 
various contributors to the public debate have increasingly expressed the opinion that class size 
does not matter. For example, the popular science author Gladwell (2013) uses class size reduction 
as an example of a “thing we are convinced is such a big advantage [but] might not be such an 
advantage  at  all”  (as  cited  in  Schanzenbach,  2014).  Likewise,  in  a  recent  report  the  OECD 
concludes that "overall, evidence of the effect of differences in class size on student performance 
is weak" (OECD, 2016, p.349). These strong statements are made without (OECD) or with very 
selective  (Gladwell)  reference  to  evidence,  let  alone  a  careful  assessment  of  the  evidence.  The 
same can often be said of advocates of class size reductions.  

Our  systematic  review  of  the  empirical  studies  on  the  effects  of  class  size  on  student 
outcomes in  Europe shows mixed evidence. Some credible studies report substantial  beneficial 
effects of smaller class sizes, whereas other equally credible studies find effects that are rather 
small or not significantly different from zero. While some studies show that smaller classes did 
indeed  offer  sustained  benefits,  using  this  evidence  for  prescriptive  policy  purposes  typically 
requires  an  extrapolation  to  a  different  population.  Such  extrapolations  require  a  solid 
understanding of how population characteristics, incentives and constraints enter the production 
function  and  mediate  class-size  effects.  While  there  are  some  studies  that  investigate  such 
mediating factors, the evidence falls short in providing definitive answers. Based on the current 
evidence it is therefore hard to give an unqualified recommendation about how and when to use 
class size policies to improve student outcomes.  

Additional research is therefore necessary to provide better answers to the questions whether, 
why, when and for whom class size matters. These questions must be answered through research 
design,  and  the  evidence  must  be  causal.  Field  experiments  are  typically  considered  the  “gold 
standard”.  While  the  single  large-scale  randomised  experiment  regarding  class  size  (Project 
STAR) has produced many insights, they have their own threats to internal and external validity 
(Heckman and Smith, 1995). We therefore believe it is important to collect evidence from a variety 
of causal designs. These can be nonexperiment designs derived from maximum class-size rules, 
or alternative experimental designs (Rockoff, 2009).  

Finally it is worth pointing out that the notion of a class-size effect depends on traditional 
modes of teaching and learning where we can think of class size as a well-defined and policy-
relevant proxy for inputs in schools’ production function. While today’s schools are remarkably 
similar to the schools of our grandparents, technology and pedagogical innovations may change 
the nature of classrooms and thereby the relationship between class size and inputs. Estimates of 
class-size effects are likely to have expiration dates when the production function is changing. 

References 
Gladwell, M. (2013). David and Goliath: Underdogs, Misfits, and the Art of Battling Giants. Little, 
Brown & Company., New York. 

Heckman, J. J. and Smith, J. A. (1995). Assessing the case for social experiments. The Journal of 
Economic Perspectives, 9(2):85–110. 

 
 
 
 
 
 
 
 
OECD (2016). Education at a Glance 2016: OECD Indicators. OECD Publishing, Paris.  

Rockoff, J. (2009). Field experiments in class size from the early twentieth century.  Journal of 
Economic Perspectives, 23:211–230.  

Schanzenbach, D. W. (2014). Does class size matter? National Education Policy Center. 

 
 
 
 
NC-02-17-767-EN-N 

doi: 10.2766/59709 

ISBN: 978-92-79-70095-8