Our results vs. those of Caldwell et al. (2011) for the 36 common massive clusters, shown for the mass (panel a)), the metallicity (panel b)). Panel c) shows the difference of metallicity between this work and Caldwell et al. (2011) values vs. the cluster mass. For panel c), error bars of Caldwell et al. (2011) and derived here are summed quadratically. Filled dots are the group 1 clusters and open dots are the group 2 clusters, as defined in Fig. 3. Clusters marked 1–5 in this figure and Figs. 5 and 6 are the same objects.