Structure function for XMM-Newton subsamples binned in intervals of log M_BH and log L_bol, with bin width Δlog M_BH = Δlog L_bol = 0.5. Values of M_BH and L_bol are reported in the upper and right axes, respectively. Subsamples with less than 30 SF points are not shown. The SF of the total XMM-Newton sample is reported, for comparison, in the first box in the upper left corner. The values reported in each box are the number of SF points, the average SF slope, and the expected value of log τ_break, according to Eq. (6). Contributions from pairs of individual measurements are also shown (dots).