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Methods A large - scale survey was conducted at 250 medical health units based on stratified sampling.
方法使用分层抽样的方法在全国范围内抽取250家医疗机构开展大规模调查.
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Methods: With stratified sampling and cluster sampling, we investigate 487 families in poverty.
方法采用分层整群抽样方法调查487户贫困家庭.
分层抽样???
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