Effect of Time Spent Outdoors at School on the Development of Myopia Among Children in China - A Randomized Clinical Trial

1,903 grade-1 children (952 intervention, 951 control) in 12 primary schools; 6 clusters per arm

Grade-1 children (mean age 6.6) in Guangzhou, China, followed 3 years (2010-2013)

Cluster RCT, 12 schools. Intervention added one 40-minute outdoor class per school day plus parental encouragement for outdoor time at weekends/holidays. Primary outcome was 3-year cumulative myopia incidence by cycloplegic refraction (REC standard, SE <= -0.5 D); ITT with mixed linear models. Only 12 clusters, so cluster-level power is modest and the trial was not masked to participants. No academic or cognitive outcome of any kind was measured.

Three-year cumulative myopia incidence 30.4% intervention vs 39.5% control (difference -9.1%, 95% CI -14.1 to -4.1, P < .001). Refraction change -1.42 D vs -1.59 D (difference 0.17 D, 95% CI 0.01 to 0.33, P = .04). Axial elongation 0.95 mm vs 0.98 mm was NOT significantly different (difference -0.03 mm, 95% CI -0.07 to 0.003, P = .07) - the biometric outcome did not track the incidence result.

None for the causal claim: allocation was randomized at school level, so heritable child and parent traits cannot differ systematically between arms. Myopia itself is among the most heritable common traits (h2 ~0.75-0.90 in twin studies), which is why only randomization is informative here.