SDSS quasar spectra. Left panel: green dashed lines indicate the positions of emission lines. The emission line redshifts z_em are determined from the [O II] emission lines. The blue symbols are the radio loudness of quasars: . Right panel: residual spectra of quasars where the continuum+iron fits have been subtracted from the original spectra. The blue and red lines are the narrow and broad Gaussian function fits, respectively. The green lines are the sum of the blue and red lines.