Differences in metallicity between our determinations and those of the literature are plotted here against [Fe/H] values computed in this work. In the upper panels the comparison metallicities are those of H10, while in the lower panels they are those of Appendix 1 of C09. The dashed and dotted lines represent the average and  ± σ of the metallicity differences, which were computed after retaining clusters with deviations smaller than 1 × σ from a preliminary fit (encircled single or double diamonds). The left panels show the 14 calibration clusters, and the right panels show the 20 program clusters (17 for the comparison with C09/Appendix 1, see Table 6). In the right panels single diamonds represent clusters with weight smaller than 3 in H10. Error bars in the lower panels represent the quadratic sum of our errors and those of C09.