English name for this index - Least Squares Method

Hi all,

I was wondering what is the English name for the index I_2, used to evaluate the goodness of the fit:

I_2= SQRT[ SUM(Y_i-y_i)^2/N]/SUM(Y_i)/N,


Y_i is the ith value of the variable to be predicted and y_i=m*X_i+q is the predicted value of Y_i. N is the number of points (Y_i, X_i).

Many Thanks,