Study Design. Prospective, observational study.
研究设计：前瞻性观察研究。
We can never be sure in an observational study of a casual relationship.
我们永远不能肯定是一个因果关系的观察研究。
The smoking situation is taken from an observational study in early 2008.
图中所示吸烟情况源自2008年初一项观察性研究。
The smoking situation is taken from an observational study in early 2008.
图中所示吸烟情况源自2008年初一项观察性研究。
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