3方差的专业定义是随机变量的二次中心矩，与期望值之差的平方的加权平均值。

The technical definition of variance is the random variable's second central moment, the weighted average of the square of the differences from the expected value.

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据美国科罗拉多大学博尔德分校国家冰雪数据研究中心的统计资料显示，今年9月的北极海冰面积比1979年至2000年的平均值低39%。

According to the National Snow and Ice Data Center at the University of Colorado at Boulder, September sea ice was 39 percent below the long-term average from 1979 to 2000.

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深圳疾病预防和控制中心副主任马汉武称，今年流感病人增加了20%，但这一数字低于五年的平均值。

Ma Hanwu, deputy director of Shenzhen Center for Disease Control and Prevention, said the number of flu patients had increased by 20 percent this year, but was still lower than the five-year average.

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