Structure function for the XMM-Newton sample in bins of Δlog τ = 0.5. The small empty circles and the continuous line connecting them show the uncorrected SF (i.e., neglecting σ_n in Eq. (1)). The larger filled circles and the line connecting them, show the SF corrected for the noise. The continuous line without data points indicates the average value of the noise in each bin, and the dashed, horizontal line is its weighted average, according to the number of points in each bin, adopted in Eq. (1). The dotted line is a weighted least-squares fit to the data of the bins. The small dots are the contributions from pairs of individual measurements at times differing by τ.