Comparison of the effective number of binaries in terms of N₇₅ and N_eff contributing to the GWB at each observed frequency bin of a PTA with an observing duration of 10.3 years as for EPTA Collaboration (2023a). At each observer frequency, the two extreme horizontal lines represent the 5th and 95th quartiles of the N₇₅ distribution, derived from 2000 Universe realizations. The edges and the central line of the box represent the 25th, 50th, and 75th quartiles, respectively. For N_eff, we only show the median of the distribution at each observer frequency with blue dots for the circular population and orange crosses for the eccentric population.