Logarithmic flux (in erg s^-1 cm^-2) versus the signal-to-noise ratio of the [O III] emission line. We adopt the dotted line as the threshold in flux and S/N to select only spectra with a visible [O III] emission line and reliable fit (green points). Red points represent the quasars for which we consider the kinematic parameters based on unreliable [O III] lines and we do not consider these QSO in the following analysis.