Methods A cluster sample of 8 054 students(aged 12-18 years) were chosen from 28 middle schools in Guangzhou.
方法采取整群抽样方法，对广州市28所中学12~18岁的中学生8 054名进行吸烟状况的问卷调查。
Methods With the method of questionnaire survey, a cluster sample of 3 012 students (aged 15-19) were investigated. They were chosen from Qinghai Province, Shanxi Province and Zhejiang Province.
方法选择青海省、山西省、浙江省各1个县，采用整群抽样的方法，共选取3 012名15 ~ 19岁在校中学生进行问卷调查。
A sample cluster topology is shown in figure 2.
图2中是一个示例集群拓扑。
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