方差定义见“概率论与数理统计(浙大四版)”P109.
样本方差的标准定义见“概率论与数理统计(浙大四版)”P145，注意是除以1/(n-1)不是1/n，尽管根据方差定义，感觉应该是除以1/n。看std的matlab帮助：There are two common textbook definitions for the standard deviation s of a data vector X.一个是除以n，一个是除以n-1。
通过除以1/(n-1)能证明样本方差是方差的无偏估计，见“概率论与数理统计(浙大四版)”P151公式(3.20)，Hanyang Peng曾经给我推过这个证明，很easy。
matlab函数var，matlab2013a帮助说明
var normalizes V by N– 1 if N > 1, where N isthe sample size. This is an unbiased estimator of the variance ofthe population from which X is drawn, as long as X consists of independent, identically distributed samples. For N =1, V is normalized by 1.
V = var(X,1) normalizesby N and produces the second moment of the sample about its mean. var(X,0) is equivalent to var(X).
second moment：二阶矩