Comparison of light-weighted ages (left) and light-weighted stellar metallicities (right) obtained for individual duplicate observations (y-axis; duplicate measurements for each galaxy are connected with a dashed line) with those obtained from the duplicate-combined indices (x-axis). Filled symbols refer to those galaxies whose fit is constrained by the full set of five optimal indices (D4000_n, Hβ, Hδ_A+Hγ_A, [Mg₂Fe], [MgFe]′) when duplicate observations are combined. The error bars indicate the mean uncertainties in each estimate, with the thicker error bars referring to estimates based on the five optimal indices.