The effects of class size

It seems logical that class size must influence the classroom experience of both teachers and students, yet researchers report that there has been a lack of good evidence on the effects of class size differences. Peter Blatchford and Clare Martin, University of London, describe the problems in existing research and identify classroom processes that are likely to be affected by class size and those that may influence student achievement. They also propose parameters for future research.

Their research addresses not only whether class size makes a difference in student achievement, but how it makes a difference. In other words, which processes within the classroom are influenced or change as the size of the class changes, and how do these processes affect student achievement?

Early studies tended to find little or no relationship between class size and student outcomes. However, a review of the research by Slavin in 1990 identified eight well-constructed studies that showed a moderate effect on achievement in favor of classes with fewer than 20 students. The recent Student Teacher Achievement Ratio (STAR) research in Tennessee has renewed interest in class size effects. Results demonstrate that small class size does have benefits, especially in the primary grades. The most solid conclusion that can be drawn from existing research is that the largest effects are seen with children who are just beginning school. There is also evidence that smaller class size is particularly beneficial for disadvantaged children.

Impact of class size

Elementary classes are larger than secondary classes in Great Britain. On average, elementary classes have 28 students, while secondary classes have 22. Average elementary class sizes have increased in recent years by 2.1 pupils per class between 1982-1996. The percentage of classes under 20 students has been cut in half, and more than 25 percent of elementary classes now have more than 30 students.

These researchers identify five factors that may be influenced by class size:

1.grouping practices within classes;

2.the nature and quality of teaching;

3.students’ attention to task;

4.students’ adjustment to school 5.teachers’ morale, stress and enthusiasm.

They suggest that changes in these areas can affect how much progress students make. They also contend that student achievement can be affected by how the teacher responds to differences in class size. Some teachers may not change the way they teach a smaller class — may not take advantage of the opportunities smaller classes offer in terms of more effective teaching practices.

On the other hand, some teachers are able to mitigate the adverse effects of larger classes through extra effort and expertise. Blatchford and Martin believe, however, that this may come at some cost to the teacher’s well-being. Although there is some evidence linking these five factors to class size and to student achievement, it is not conclusive.

Obstacles to research evidence

Several things stand in the way of good research evidence on the effect of class size on student achievement. First, much more accurate information is needed on pupil/teacher ratios. Researchers must collect data that validly describe students’ actual experience by tracking the average size of a child’s classes over time. Dividing the number of students by the number of professional staff in a building does not give an accurate picture of the number of students in regular classrooms.

Second, reliance on experimental data as the main source of evidence on causal effects of class size differences is questionable, in these researchers’ opinion. It is their belief that correlational and observational analysis of naturally occurring differences in class sizes and pupil/teacher ratios without intervention can be more valid and useful. Studies should be long-term, with baseline data collected and follow-up with the same students over several years. Sophisticated statistical techniques such as multilevel modeling should be used to capture the complex structure of educational data.

Third, more study of the mediating factors between class size and academic achievement is needed. Blatchford and Martin’s ongoing research project is to help generate and test theories on the pedagogical effects of class size differences.


“The Effects of Class Size on Classroom Processes: It’s a Bit Like a Treadmill — Working Hard and Getting Nowhere Fast!” British Journal of Educational Studies, Volume 46, Number 2, June 1998, pp. 118-137.

Published in ERN September 1998 Volume 11 Number 6

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