DESIGN: a cluster sampling and simple randomized sampling survey.

设计：整群抽样与单纯随机抽样。

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Methods: This study adopted simple randomized controlled design.

方法：本研究采用单纯随机对照研究设计。

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Methods: This study adopted simple randomized controlled design.

方法：本研究采用单纯随机对照研究设计。

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