世界平均人口密度为每平方英里47人。
The average population density of the world is 47 persons per square mile.
而美国每年的2000宗飞碟目击事件平均到每平方公里只有0.0002起。
The 2,000 UFOs are spotted every year in the United States represent just 0.0002 sightings per square kilometer.
然后你减去e的平均值,这个量再平方。
So you see what it is--it's a measure of how much x deviates from the mean, but it's squared.
你会看到,这个指标,衡量的是x与平均值的偏离,而且这里有个平方
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