Reducing class size in the earliest years produced measurable attainment gains in a randomised trial.
Project STAR randomised pupils and teachers to small and regular classes across Tennessee from 1985, and found gains for the small classes that persisted for several years. California's later statewide reduction found little, in a setting where the policy itself changed who was teaching.
In practice
- A trial that holds teacher quality constant and a policy that cannot are measuring different things.
- A school that halves class sizes by hiring in a shortage may have traded the effect it was buying.
- Possible explanations
- One explanation is attention per pupil, which rises as numbers fall. Another is behaviour, since a smaller room is easier to manage. A third is teacher supply, which explains why the result weakens at scale: a state that halves class sizes must hire many more teachers, and the additional ones are less experienced.
- In its favour
- Project STAR randomised pupils and teachers, which few education studies manage, and found gains in the earliest years that persisted for several years afterwards.
- Against it
- Statewide reductions elsewhere produced much smaller or no gains, the cost per point of attainment is high compared with other interventions, and the teacher-supply constraint means the trial result may not transfer to policy.
- Standing
- The STAR result is among the strongest evidence in education and has not generalised cleanly: statewide reductions elsewhere produced much smaller effects, partly because they forced hiring of less experienced teachers.
- Subject
- Learning & Education
- Essay
- What works in learning, and what schools can and cannot change