The Number of Hits fitness function is very simple and highly efficient, and is based on the number of samples correctly classified.

More formally, the fitness f_i of an individual program corresponds to the number of hits and is evaluated by the formula:

f_i = h

where h is the number of fitness cases correctly evaluated (number of hits).

So, for this fitness function, maximum fitness f_max is given by:

f_max = n

where n is the number of fitness cases.

Its counterpart with parsimony pressure, uses this fitness measure f_i as raw fitness rf_i and complements it with a parsimony term.

Thus, in this case, raw maximum fitness rf_max = n. And the overall fitness fpp_i (that is, fitness with parsimony pressure) is evaluated by the formula:

where S_i is the size of the program, S_max and S_min represent, respectively, maximum and minimum program sizes and are evaluated by the formulas:

S_max = G (h + t)

S_min = G

where G is the number of genes, and h and t are the head and tail sizes (note that, for simplicity, the linking function was not taken into account). Thus, when rf_i = rf_max and S_i = S_min (highly improbable, though, as this can only happen for very simple functions as this means that all the sub-ETs are composed of just one node), fpp_i = fpp_max, with fpp_max evaluated by the formula: