Upper panel: histograms of the V (black unmarked line), V_sys2σ (red line with circles), and V_sys3σ (blue lines with triangles) parameters; lower panel: plot of V_sys2σ vs. the Reduc.F_sys. The vertical line in the upper plot is the threshold above which a source is variable (V ≥ 1) according to the V parameters. The horizontal line in the lower plot has the same meaning, while the vertical dashed line is the approximated extrapolation of a threshold for source variability for the Reduc.F_sys parameter, taking into account the correlation of both parameters.