Distribution of the galaxies in the Hδ_A+Hγ_A vs. D4000_n and Hδ_A+Hγ_A vs. [MgFe]′ planes before and after the implementation of the statistical weights and the aperture corrections. Left panels: Distribution of the original data, i.e. simple number counts for uncorrected index values. Central panels: Distributions of galaxies weighted by the volume and completeness statistical corrections, for uncorrected index values. Right panels: Distributions obtained by implementing both the statistical weights and the corrections for aperture effects. The error bars correspond to the median uncertainties calculated in four bins of D4000_n and [MgFe]′, for samples with S/N ≥ 20 (red) and S/N ≥ 10 (shown in black at the top).