The mean absolute residuals in [Fe/H] as a function of a_R for stars in different age bins (different colors); median errors on the mean for each age bin are shown at the bottom. The dashed curves show the predictions of Equation (B1) with m_R = −0.0678 dex kpc⁻¹ and the best-fit value of σ_tot for each age bin; the shaded regions indicate the range of model curves obtained by varying the fitted σ_tot by ﹩\pm 1{\sigma }_{{\sigma }_{{\rm{tot}}}}﹩.