The intervention group undertook interventions by increasing time spent outdoors. The interventions composed of performing two additional recess program lasting 30 minutes outside the classroom that encouraged children to go outside for outdoor activities during recess in both the morning and afternoon during school days within a intervention period of 1 school year.

The control school did not have any interventions.

The intervention group undertook interventions by increasing time spent outdoors. The interventions composed of performing two additional recess program lasting 30 minutes outside the classroom that encouraged children to go outside for outdoor activities during recess in both the morning and afternoon during school days within a intervention period of 1 school year.

Uncorrected visual acuity (UCVA) was measured for the right eye, then the left eye, with a E Standard Logarithm Vision Acuity Chart (GB11533-2011) in 5-grade notation (5- LogMAR), with illumination of the chart around 500lx. If one of the eyes UCVA was less than 5.0, then tests with combined lenses were conducted among the poor eye-sighted for confirming the types of ametropia (normal, myopia, hyperopia or other eye diseases)．All the measurements were recorded.

The parents and children underwent an interview together. The questionnaire included questions on children's gender, ethnicity, region of habitation; questions on parental education level, the monthly family income and whether the parents were myopic or not;. The interview also included questions on the amount of time spent for learning (reading or writing), screen time (television, computer, etc) and time spent outdoors after school in recent 7 days.