Fits to SDSS data. Top: Measured iron metallicity values as a function of stellar velocity dispersion for the SDSS stacked spectra. We recover the well-known relation between the metallicity of galaxies and their velocity dispersions based on the linear combination of MILES stars. Note that metallicity refers here to [Fe/H] as it is based on the metallicity of these individual stars. Bottom: Ratio between the weight given to low-mass and high-mass stars by our fitting approach. This weight ratio is a proxy for variations in the IMF and suggests a relative excess of low-mass stars in the most massive ETGs.