Study question

Setting: 30 primary schools in Guangzhou, China.

Patients: Students underwent annual visual acuity tests. Schools were stratified to six strata based on the proportion of students with ‘normal’ vision (uncorrected visual acuity >20/25) in grades 1–6. Two schools were randomly selected from each strata and all grade 1 students (age 6 years) enrolled in the study; one school to intervention, the other to control.

Exclusions: Any student with pre-existing myopia, systemic or ocular disease.

Intervention: Compulsory additional 40 min exercise class after school; parental consent not required for participation. Students in control group continued their usual daily routine.

Outcomes: Primary outcome was the incident rate of myopia, defined as refractive error of −0.5 D or worse …