Dependence of the error in obtaining the gravitational binding energy by sampling, rather than by direct summation over all the pairs of particles, with the fraction n_sample/N (upper panel) and with just the number of sample particles n_sample (lower panel). The number of evaluated pairs of particles is . Different colours, according to the legend in the lower panel, represent different number of particles within the halo. The shadowed regions correspond to (16 – 84)% confidence regions.