Left panel: comparison of the metallicity from high-resolution spectra for five RR Lyrae stars in common in Nemec et al. (2013) and Crestani et al. (2021). Right panel: comparison of the photometric metallicities derived by the SOS pipeline and the spectroscopic metallicities from high-resolution spectra derived by Crestani et al. (2021) for 105 RR Lyrae stars in common in the two samples. The two dashed lines are offset by ±0.46 dex, the mean error of the photometric metallicities (see Fig. 19), from the one-to-one line.