Notes. The columns show: the selected sample; the number of fitted data points; the total χ² value; the intrinsic scatter estimated by requiring that the reduced χ² is equal to 1; the best-fit parameters of the adopted functional form Z(r,z) = Z₀(1 + (r/ 0.15 R₅₀₀)²)^{− β}(1 + z)^{− γ}. Median, 1st and 3rd quartiles from the distribution of the best-fitting parameters in the bootstrap analysis over a sample of 10⁵ repetitions are indicated in round parentheses. In the last column, labelled “constant”, we list the weighted-mean, its relative error and the dispersion around it in round parentheses.