MSE explicit

Manuscript/QUEST_WIREs.tex

@@ -719,7 +719,7 @@ And $\err_\meth$ the error vector of the method $\meth$ compared to the referenc
 When the vertical excitation $x$ is defined for the method $\meth$ and the method $\text{ref}$.
 So with $\nExnn$ the size of the vector $\vec{\err^x_\meth}$
 \begin{gather}
-	MSE_\meth =  \overline{{\vec{\err_\meth}}} \\
+	MSE_\meth =  \overline{{\vec{\err_\meth}}} = \frac{1}{\nExnn}\sum_x=1^\nExnn\err_\meth^x \\
 	MAE_\meth =  \overline{\abs{\vec{\err_\meth}}}  \\
 	RMSE_\meth = \sqrt{\overline{\vec{\err_\meth}^2}} \\
 	SDE_\meth = \sqrt{\frac{1}{\nExnn}\sum_{x=1}^\nExnn\err_x^2-MAE^2}