Fractional EW variations for the 28 non-disappearing BAL troughs in the ND sample. The two epochs used for each trough are the same as in Fig. 7. The horizontal axis shows the offset of the central velocity υ_cND of each non-disappearing trough with respect to the υ_c of the corresponding disappearing trough. The central velocity for a trough is defined as υ_c = (υ_max + υ_min)/2; error bars are computed propagating the errors on the EWs in the two epochs. The horizontal and the vertical dashed lines represent the zero levels for each axis, and are plotted for a better identification of the various regions in the diagram.