The prevalence and associated risk factors of myopia in grade 1 and grade 4 primary students of defined schools in urban and rural regions of Beijing
GUO Yin, LIU Li-Juan, XU Liang, LV Yan-Yun , TANG Ping, FENG Yi, MENG Meng
Ophthalmology in China . 2012, (2): 127 -131 .