The Journal identified more than 100 middlemen - tracking companies, data brokers and advertising networks - competing to meet the growing demand for data on individual behavior and interests.

《华尔街日报》指出在这之间存有百余个中间商，其中包括跟踪公司、数据代理和广告网络，他们竞相提供大量的关于个人行为与兴趣的数据，以满足日益增长的需求。

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That's because, in a sparse data set, no individual person whose behavior is being recorded is likely to exhibit more than a limited range of behaviors.

这是因为在小型数据集当中，除非其表现超出预先设定的具体范围，否则个人行为是不会被记录下来的。

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Today, as some of the best minds of Silicon Valley crunch enormous volumes of data being mined by Google and others to monitor the online behavior of individual users, advertising has entered a new age.

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