Methods Observational study.

方法观察性分析。

Study Design. Prospective, observational study.

研究设计：前瞻性观察研究。

METHODS: We performed a cross-sectional observational study.

我们进行了一个横断面观察性研究。

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。