Scatter σ_eva, 0 as a function of n_eva, 70. The clusters are binned according to their n_eva, 70 so that each bin has ten or more clusters, with the exception of the last bin that has 8 clusters. The center of each bin is the mean of n_eva, 70 of the clusters in that bin. ϕ_eva and σ_eva are then derived for each bin using Eqs. (1) and (3). The least-squares fit to the [bin center, σ_eva] pairs (blue points) is done in log–log space (see the inset panel), weighing by the square root of the number of clusters in each bin, which results in a fit f(x) = 1.5 x^−0.8 in linear space (red line).