[方差 协方差]方差 variance协方差 covariancehttps://en.wikipedia.org/wiki/Variance方差 一个随机变量的方差描述的是它的离散程度,一个实随机变量的方差,也成为它的二阶矩或二阶中心动差.Informally, it measures how far a set of (random) numbers are spread out from their average value. Variance has a central role in