where P_(ij) is the value predicted by the individual program i for sample case j (out of n sample cases); and T_j is the target value for sample case j.

For a perfect fit, P_(ij) = T_j and E_i = 0. So, the E_i index ranges from 0 to infinity, with 0 corresponding to the ideal.

To evaluate the MSE of your model both on the training and testing data, you just have to go to the Results Panel after a run and, although it is not shown there, it is also evaluated there and kept for your future reference in the Report Panel.