Light curve r.m.s. σ^lc as a function of the average magnitude ⟨mag^lc⟩ for all the sources in the main sample (small gray dots). The dashed line represents the running average of the r.m.s., while the solid line defines the variability threshold. Objects above the threshold are considered to be variable, and large red dots identify non-spurious AGN candidates (flag 1 or 2, see further in the text).