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

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

As it stands, E_i can not be used directly as fitness since, for fitness proportionate selection, the value of fitness must increase with efficiency.

Thus, for evaluating the fitness f_i of an individual program i, the following equation is used:

which obviously ranges from 0 to 1000, with 1000 corresponding to the ideal (the coefficient 1000 allows a fairer distribution of fitnesses for selection).