现在才弄懂为什么用n-1....
计算方差，有时候用 1n∑ni=1(Xi−X¯)2,有时候用1/(n-1),这是为什么？
第一个叫“population variance",第二个叫”sample variance" 。
例如城市里有10000个人，你测量这10000个人的身高，之后计算方差，用第一个公式，因为这里10000人，不是抽样(sample),而是整体(population)。
如果你只抽样1000人，那你就用第二个公式。
为什么？这我不完全明白 As a result, the calculated sample variance (and therefore also the standard deviation) will be slightly higher than if we would have used the population variance formula. The purpose of this little difference it to get a better and unbiased estimate of the population‘s variance (by dividing by the sample size lowered by one, we compensate for the fact that we are working only with a sample rather than with the whole population).

这个有心情我算算看，暂不排上日程了。

参考：
http://www.macroption.com/population-sample-variance-standard-deviation/