来自《耶鲁大学开放课程：心理学导论 第10集 第10集》

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  as some people quickly noted, two percent.
  正如有人很快给出的 百分之二
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  And here's how it works.
  事情是这样的
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  One percent of a thousand will have the disease.
  得这个病的概率是千分之一
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  That person will test positive.
  这个人会被检测为阳性
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  The test never misses.
  测试不会有遗漏
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  That leaves nine hundred ninety-nine people who don't have the disease,
  这样就剩下九百九十九个人没得这种病
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  and we'll say about fifty percent
  我们说有百分之五的人
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  of these people have it.
  可能也会被检测出来有
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  So, for every fifty-one people who test positive,
  这样每五十一个测试者中
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  only one will have the disease,
  只有一个真正得了这种病
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  giving an average of about two percent.
  得出平均几率大约是百分之二
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  This sort of thing is very difficult.
  这种推理过程是非常困难的