r_s (R ^1,R ^2) = \frac{\sum_{i = 1} ^n (R_i ^1 - \overline{R ^1}) \cdot (R_i ^2 - \overline{R ^2})}{\sqrt{\sum_{i = 1} ^n (R_i ^1 - \overline{R ^1})^2 \cdot \sum_{i = 1} ^n (R_i ^2 - \overline{R ^2})^2}} = \frac{S_{R ^1} + S_{R ^2} - \sum_{i = 1} ^n (R_i ^1 - R_i ^2)}{2 \sqrt{S_{R ^1} \cdot S_{R ^2}}}