In comparing the prevalence of childhood myopia from extremes in environmental settings (rural versus urban), distinguishing effects of urbanization from the impact of other factors (e.g., education, schooling, and outdoor activity) can be difficult, so that it is not always entirely clear whether differences in myopia prevalence can be attributed to an urban environment alone. Among 12-year-old school children in China, one prevalence estimate of myopia in an urban sample was 49.7%,
15 but it ranged from 12.0% to 23.0% in semirural samples.
17 In a study of younger children in China attending school in the city or in the countryside, Saw et al.
18 reported that the proportions with myopia were 19.3% and 6.6%, respectively. In these younger children, there were also substantial differences in level of near-work activities, in the proportions with parental myopia and in the level of parental education, so that differences in myopia prevalence could not be attributed to urbanization or geographic region alone. In another study of children in Malaysia and Singapore,
23 ethnicity-specific comparisons consistently showed higher rates of myopia in Singaporean children. Although the authors proposed that environmental differences such as early schooling, population density, and apartment housing may have attributed to the differences in myopia prevalence between the two countries, specific analyses of such factors were not reported. Lithander,
22 in a survey of myopia prevalence in a sample of children from the Sultanate of Oman, found that children in rural remote areas were significantly less myopic than the rest of the sample. This finding was attributed to the high illiteracy rate and the low level of reading or writing undertaken by inhabitants in the rural areas. In a sample of rural school children in China, He et al.
16 found that school locality (urban school versus rural school) was significantly associated with myopia and was thought to be the result of various levels of near work and outdoor activity. These factors, however, were not reported.