Geometric mean composite spectra at bright and faint epochs, represented in blue and red colors, respectively. The gray and green curves are composite difference spectra for all sources, and for sources with both bright and faint epochs in DR7, respectively. All composite spectra were normalized at 2250 Å, but the composite spectrum of the faint epoch (red) and the composite difference spectrum (gray and green) were multiplied by 0.76 and 0.30, respectively, to clearly separate them from that of the bright epoch. The black solid lines are power-law fits to the continuum. The upper small panel shows the number of objects in each wavelength bin (1 Å) when constructing the corresponding composite spectra.