Method: Using stratified cluster sampling, sampling an elementary school respectively in the city and countryside, carries on the check by sight and the correlation factor investigation.
方法：应用分层整群抽样方法，在城市、农村小学中各抽取一所小学，进行视力检查及相关因素调查。
Method: Using stratified cluster sampling, sampling an elementary school respectively in the city and countryside, carries on the check by sight and the correlation factor investigation.
方法：应用分层整群抽样方法，在城市、农村小学中各抽取一所小学，进行视力检查及相关因素调查。
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