Average metallicity and associated rms scatter for NGC 6388 resulting from the present work (the blue circle on the right indicates the value from UVES spectra, the red one is for GIRAFFE spectra) and from literature studies. The rightward point for L13 refers to the analysis made with EWs (and not with the CaT). The rightward point for M20 refers to the value obtained considering only the high S/N spectra and the rightward point for H20 is derived from their HB method (see text).