Histogram of the observed (O_λk) minus model (M_λk) residuals at each wavelength normalized by the formal flux error (ϵ_λk). The model spectra are obtained from Starlight fits for the stellar continuum, excluding the main emission lines, as well as bad pixels. The histogram describes almost a Gaussian centered at 0 with a dispersion of  ~0.8. The blue line indicates a Gaussian distribution centered at 0 with a dispersion of 1, normalized to the total number of pixels for comparison. On the assumption that the spectral residuals are due to noise (i.e., neglecting model imperfections), this figure demonstrates that the pipeline error estimates are reliable.