eBASE, effective basic services:Reducing Class Size

Reducing Class Size

Low impact for very high cost based on very limited evidence
Implementation cost
Evidence strength
Impact (months)

Reducing class size is an approach to managing the ratio between pupils and teachers, as it is suggested that the range of approaches a teacher can employ and the amount of attention each student will receive will increase as the number of pupils per teacher becomes smaller.

1. Reducing class size has a small positive impacts of +2 month, on average. The majority of studies examine reductions of 10 pupils. Small reductions in class size (for example, from 30 to 25 pupils) are unlikely to be cost-effective relative to other strategies.

2. There is some evidence for additional benefits of smaller class sizes with younger children, so smaller class sizes may be a more effective approach during the early stages of primary school. 

3. Smaller classes only impact upon learning if the reduced numbers allow teachers to teach differently – for example, having higher quality interactions with pupils or minimising disruption.

4. The gains from smaller class sizes are likely to come from the increased flexibility for organising learners and the quality and quantity of feedback the pupils receive (see Feedback).

5. As an alternative to reducing class sizes, it may be possible to change the deployment of staff so that teachers can work more intensively with smaller groups (see Small group tuition).

The average impact for reducing class size is around 1 month additional progress over the course of an academic year. The evidence in this area of very limited, so should be treated with caution.

The key issue appears to be whether the reduction is large enough to permit the teacher to change their teaching approach when working with a smaller class and whether, as a result, the pupils change their learning behaviours. If no change occurs then, perhaps unsurprisingly, learning is unlikely to improve. When a change in teaching approach does accompany a class size reduction (which appears hard to achieve until classes are smaller than about 20) then benefits on attainment can be identified, in addition to improvements on behaviour and attitudes.

In Sub-Saharan Africa (SSA), the research evidence pertaining to the effects of reducing class size on the academic achievements of learners is varied. While some studies show positive effects of reducing class size and learners’ outcomes, others have produced null findings, and in some cases, negative effects.

In an analysis of national cross-sectional data collected by the UNESCO Institute of Statistics (UIS), it was established that as the number of students per teacher increases, the chances of primary school completion drops. Reducing the number of students per teacher may free resources. This will allow more teacher-student time and hence greater focus on individual student learning preferences, potentially resulting in greater academic achievement for students, including those at risk. Similarly, an analysis of the effects of school resources on students’ mathematics achievement in Zimbabwe established that well-trained teachers improved test scores, although student achievement is negatively affected if teachers are made to teach large classes”.

The positive associations between class sizes and educational outcomes may be a result of where students find themselves. A study investigating the dependence of the relationship between class size and learner outcomes on the school socio-economic status (SES), suggests that the effectiveness of class size reductions in improving learner outcomes may be dependent on other characteristics of the school such as teacher quality and school functionality. This implies that although reduced class sizes could be one way to improve learner outcomes, it may not be if other factors in relation to school and learning quality are not addressed. A program under which randomly selected Kenyan schools were funded to hire additional teachers on an annual contract at one quarter normal compensation rates revealed no significant increase in test scores for students randomly assigned to existing classes, despite a 46% reduction in class size. In contrast, the scores of locally-hired contract teachers increased, though the authors suggest this may be due to the low absence rate of the contract teachers and a reduced effort of centrally-hired teachers in schools where locally recruited teachers were randomly assigned.

While the global evidence suggests moderate impact of reducing class size on educational attainment, the local evidence in SSA is mixed. Even where positive associations are established, the extent to which class size reduction improves attainment is in dispute and some effects can only be seen when certain variables are controlled for within experiments.

  • Effects are similar for both primary and secondary schools.

  • Impact on reading is higher (+2 months) than mathematics (+1 month).

  • Most studies examine reductions of eight to ten pupils. The impact of studies that examine reducing class sizes by five pupils is smaller, on average.

The evidence suggests that significant effects of reducing class size are not seen until the number of pupils has decreased substantial (to fewer than 20 or even 15 pupils). Crucially, a reduction in class size is only likely to be effective if it permits teachers to change their teaching approach to the extent that this changes the learning behaviours of pupils. High quality implementation of reducing class size might consider:

  • Additional opportunities to provide feedback on pupils
  • Time for high quality interaction between pupils and teachers e.g. modelling approaches closely with pupils.

The costs associated with reducing class size is likely to be very high. Given that it involves hiring more teachers and, commonly an expansion of the existing school infrastructure, the costs of implementing reduced class size are likely to be very high.

The security of the evidence around reducing class size is rated as very limited. 45 studies were identified. Overall, the topic lost three additional padlocks because:

  • A large percentage of the studies are not randomised controlled trials. While other study designs still give important information about effectiveness of approaches, there is a risk that results are influenced by unknown factors that are not part of the intervention.
  • A large percentage of the studies were not independently evaluated. Evaluations conducted by organisations connected with the approach – for example, commercial providers, typically have larger impacts, which may influence the overall impact of the strand.
  • There is a large amount of unexplained variation between the results included in the topic. All reviews contain some variation in results, which is why it is important to look behind the average. Unexplained variation (or heterogeneity) reduces our certainty in the results in ways that we have been unable to test by looking at how context, methodology or approach is influencing impact.

In SSA, most of the studies conducted on the effects of class size on educational attainment are literature reviews and observational studies. There is one relevant randomized evaluation and two meta-analysis. With limited available evidence more randomized controlled trials are recommended.

As with any evidence review, the Toolkit summarises the average impact of approaches when researched in academic studies. It is important to consider your context and apply your professional judgement when implementing an approach in your setting.

Evidence strength
Number of studies41
Review last updatedJuly 2021