Common sense suggests that a reduction in the number of students per teacher would positively influence educational outcomes.

The argument goes that educators can spend more time focused on individuals’ learning in a classroom with fewer students.

Yet, internationally, larger class size has repeatedly been shown to correlate positively with educational achievement, suggesting that larger classes may be better.

Further, studies in the US, the UK and Germany, to name a few, have shown inconsistent results on the association between class size and student achievement, especially when making only minor reductions.

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Over the past 20 years, there has been an international decrease in the average class size in countries participating in the Trends in International Mathematics and Science Study (TIMSS). This trend holds true particularly in countries that have historically had larger classes.

Class size debates

For example, from 1995 to 2016, high-performing South Korea reduced its average class size from 51 students to 32, Thailand from 46 students to 34, and Hong Kong from 39 students to 30.

Also, Slovenia - a country that showed very strong improvement in mathematics and science over the same time period - reduced class size from 25 to 17 students.

The lack of strong evidence demonstrating a positive link between class size and achievement has led some national governments and influential policy bodies, such as the Organisation for Economic Cooperation and Development (OECD), to conclude that reducing class size is an inefficient and wrong-headed approach.

These critics of reducing class size are further empowered by the fact that many high-achieving Asian countries continue to perform well in spite of having larger classes than most other participating countries.

Complex issue

However, like many things in education, the issue is much more complicated. Although it is true that Asian countries do have larger classes when compared with their European counterparts, so do many low-achieving counties. For example, of the countries that participated in the last cycle of TIMSS at the 8th grade, one of the lowest-performing countries, South Africa, had the largest average class size of 47 students.  

Further confusing the issue, Singapore - the second-highest-achieving country - had about the same class size (37 students) as Botswana and Thailand, which both performed below the international average.

Looking at differences within countries, the topic is even more complicated. In general, smaller classes are usually found in smaller schools, which tend to be rural. TIMSS data also shows that, in some countries, smaller classes tend to have more second-language students, who historically perform lower - for example, in Bahrain, Oman, Qatar or the Quebec province of Canada.

Pupil populations

In other countries, smaller classes have a disproportional amount of lower-performing economically disadvantaged students, suggesting that the story about class size is more complicated than the OECD and others claim.

Data from international research cooperative the IEA shows that there is not a universal story to be told when it comes to class size. National policies, cultural practices and demographic differences all influence the overall relationship.

This should not be surprising. We know that educational systems around the world are designed differently and, although there is much we can learn from each other, there are rarely universal truths.

In the absence of a universal story, IEA data shows that, in some countries, reducing class size appears to be associated with high educational achievement. Further, although some of the top-performing countries in the world tend to have larger classes, over time, they have worked to decrease their numbers while maintaining high results.

In other countries, such as Norway, we actually see an increase in class size with little movement in their trend scores over time.

Results from international assessments are important because they uncover this complicated story, providing evidence that class size’s relationship with achievement is best contextualized and interpreted at the national level.  

David Rutkowsi is an associate professor with a joint appointment in educational policy and educational inquiry at Indiana University Bloomington. Dr Dirk Hastedt is executive director at the IEA, an international cooperative of national research institutions, government research agencies, scholars and analysts working to evaluate, understand and improve education worldwide