Instrumental metallicity distribution for IMACS data. When all the BRC and FRC in whole RV regime (upper left panels) are selected, the [Fe/H]_Ins distribution plotted in upper right panel does not show a statistical difference. Nevertheless, when a selection is done in the same way as Fig. 6 (lower left panels), a shift in the [Fe/H]_Ins distribution is observed in the lower right panel. The star number is also indicated in each case, following the key color adopted in this paper and the corresponding selection shown in the left panels.