Summed spectrum of the quasar J022836.08+000939.2 (black line). Top panel: DR7 spectrum. The five spectra below the top panel are the same spectral data but corrected in different ways. The continuous coloured line in each panel shows the best-fit power-law continuum to that spectrum, for different flux correction models. The text in each box shows the fitted spectral index and the best-fit χ² (and its normalised value). The grey shaded areas illustrate the fitting regions. The red dots indicate pixels removed during the power-law fitting procedure. See Sect. 4.2 for details. The six spectral panels all show that a single power law fits well. Lowest panel: six correction functions in slightly more detail.