来自《耶鲁大学开放课程：有关食物的心理学、生物学和政治学 第12集》

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  so let's just take stress.
  就拿压力来说吧
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  Let's say women are under higher stress than men,
  假设女性比男性压力更大
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  women smoke more because of the stress,
  女性因压力大而吸更多的烟
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  and they're dying of heart disease
  尔后她们死于心脏病
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  because of the stress not the smoking,
  而这个心脏病是压力而不是吸烟引起的
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  the smoking is just in there as an incidental variable.
  吸烟不过是副产品罢了
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  Now that's not the case:
  其实这不是事实
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  smoking is a very strong predictor of heart disease;
  吸烟的确很可能引起心脏病
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  but you can get a sense of
  这么说只是要让你们明白
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  the weakness of cross-sectional designs.
  这种横向研究设计的弱点
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  These are the quickest way, and the most inexpensive way
  但这是最快最省钱的方法
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  although not inexpensive in an absolute way
  尽管不是绝对地省钱