a) Black line: mean (stacked) normalized spectrum of 98291 zones from 107 CALIFA galaxies, obtained by averaging O_λ/O_λ5635. Green, red, and blue lines (offset for clarity): mean synthetic spectra for bases GM, CB, and BC, respectively. The corresponding (O_λ − M_λ)/O_λ5635 residual spectra are shown with the same colors, but they are hardly distinguishable. b) Zoom of the spectral residuals in the top panel, with emission lines removed for clarity. Dashed lines mark residuals of − 3, 0, and + 3%. c) Fraction of the 98291 zones contributing to the spectral statistics in the midde panel.