Left: Relation between the 2–10 keV excess variance (σ_NXS²) (obtained with time bins of 100 s) and the black hole mass (M_BH) for the sample of 134 objects. Filled cyan circles show full detections; red down-pointing triangles show upper limits; the grey shaded area shows the confidence interval at 1σ for the best-fit log-linear relation. Blue stars represent the median value of the M_BH and the excess variance in six M_BH bins, chosen so that in each bin there are at least 20 objects. Blue error bars indicate the bin width. Blue stars are placed solely to guide the eye, as the fit is performed on the entire sample. The p-value of the relation and the total scatter δ_tot are displayed, together with the best fit result. The best fit is reported on the plot as $y-\overline{y}=a(x-\overline{x})+b\prime$, as the fit is performed by centering the variables at their average values $\overline{x}$ and $\overline{y}$. Therefore, the true intercept of the relation is to be recovered as $b=b\prime +\overline{y}-a\overline{x}$. Right: Same as the left panel but for the RM sample. The relation is steeper but consistent with what is found for the whole sample.