In an excellent, comprehensive paper on the subject, Ed Glaeser and his compatriats trace the level of metropolitan segregation alongside changes in housing prices by race.

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"Eventually you reach a high performance level as each time you trace a word, the pattern will start to be remembered, " said Dr Zhai.

BBC: The software is based on pattern recognition

But with emerging technologies, raw biogas can be stripped of carbon dioxide and other trace gases, bringing it up to the quality level of natural gas.

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The FSA was using the 1% threshold "as a pragmatic level to determine the difference between gross and trace contamination", she said.

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