Mean cluster metallicity determined by different authors from spectra with different resolution versus reference [Fe/H]. High resolution: R ≥ 25   000, medium resolution: R ≥ 13   000, low resolution: R ≈ 1000, spectroscopic indices. All spectra have high S/N. The reference [Fe/H] values are given in Table 9. The solid line is the one-to-one relation. The bars below the legend represent the average standard deviations of the cluster means for high, medium, and low resolution from left to right, respectively. Metallicities and references for medium- and low-resolution studies are given in Table 9. Metallicities and references for high-resolution studies are given in Table 4.