a) Comparisons between observed and modelled mass (abcd) and star number (efgh) distributions as a function of [α/Fe]. Plots a) and e) show our calibration sample (solar vicinity) and panels (bcd) and (fgh) the APOGEE survey. The solar vicinity observed distribution in panels a) and e) has been corrected to account for the lower sampling of old populations in the Galactic plane. From top to bottom, models have been convolved with increasing dispersion (0.01, 0.015, 0.020, and 0.025 dex) to account for the difference in data quality. For comparison with APOGEE data, models have been shifted by +0.025 dex to allow for an offset in the α-element abundance scale between our calibration sample and APOGEE.