Fractional FWHM (left) and σ_l (right) uncertainties for the SE (black points) and coadded spectra (red points) as a function of the S/N of the line. The S/N is measured per angstrom in an emission-line-free continuum window near rest frame 1700 Å. The solid black line is the expected reference relation, ﹩{\sigma }_{V}/V\propto ﹩ ﹩{({\rm{S}}/{\rm{N}}\sqrt{\mathrm{EW}/{W}_{C}})}^{-1}﹩. See Section 3.2 for details.