We define also the sample variance \sigma^2 of all mn individual experiments as \begin{equation} \sigma^2=\frac{1}{mn}\sum_{\alpha=1}^m\sum_{k=1}^n (x_{\alpha,k}-\langle X_m \rangle)^2. \tag{10} \end{equation}
These quantities, being known experimental values or the results from our calculations, may differ, in some cases significantly, from the similarly named exact values for the mean value \mu_X , the variance \mathrm{Var}(X) and the covariance \mathrm{Cov}(X,Y) .