Median values of the difference between observed and true metallicity as a function of true r-band light-weighted age values. In both left panels, black error bars denotes case 3 ([BL3580, Fe3619, Hδ_F, Hγ_F, G_band4300, Fe4383, Ca4455, Fe4531, Hβ]) at S /N = 10, while the red dashed error bars indicates case 1 ([Hβ, Mgb, Fe5270, Fe5335, Fe5406]) at S /N = 20 (upper left panel) and case 2 ([Hβ, Mgb, Fe5270]) at S /N = 30 (lower left panel). The green dashed line in the left panels represents the 0 value of the y-axis. The blue curves in the right panels represent an example of a rest-frame template with 3 Gyr and solar metallicity. The black filled zone is the spectral range considered in case 3, while the red filled zone marks the spectral range considered in cases 1 and 2 (upper right and lower right panels, respectively).