Probability distributions for the normalised anisotropy power spectrum coefficients. Solid curves show the distributions of the multipole ratios for l ∈ {1, 2, 3, 10, 20, 40}, where lighter colours indicate higher multipoles, across four models at the first PTA frequency band. The dashed curve represents the distribution of the sum of the fractional energy density squared of all GW-emitting sources, C_∞/C₀.