Sample covariance
= Sample covariance
For paired scalar observations $(x_i,y_i)$, their sample covariance is $n^{-1}\sum_i(x_i-\bar x)(y_i-\bar y)$, or the same sum divided by $n-1$ under the unbiased convention. It is an off-diagonal entry of the <sample covariance matrix> of the paired vectors.