Source-dependent individual vs ridge metallicities (FEH, averages computed from all sources). Black line shows the ridge, blue line shows the fit to the particular source averages (assuming only a constant shift relative to the ridge line for a given source). Black dots are for the average metallicities, pale, larger circles are for the individual metallicities. The left and right panels show, respectively, the well-fit case of Luo et al. (in prep.) (LAMOST DR5) and the poor-fit case of Gilligan et al. (2021). The latter is not included in our final HDS sample. Table 1 gives the list of the HDS sources.