Top panel: structure function of APM 08279+5255; small dots represent the UDSF points (Eq. (6)) relative to BAL variations; filled circles connected with continuous line represent the corresponding binned SF; blue and red open circles connected by continuous lines represent the binned SF for blue and red NALs (blue and red, respectively). The relevant fitting relations corresponding to Table 2 are shown as dotted lines. Middle panel: ensemble structure function based on data from the literature (Barlow 1993; Lundgren et al. 2007; Gibson et al. 2008); UDSFs with different symbols as specified in the inset; filled circles connected by continuous line represent the binned ensemble SF and dotted line shows the best fit relation of Table 2. Bottom panel: structure functions of the two BAL components of APM 08279+5255; BAL₁ blue line, BAL₂ red line, for comparison black filled circles and the connecting line show the structure function of the total W_BAL as in the top panel.