Abundance ratios of [α/Fe] as a function of [Fe/H] predicted by our two-infall chemical evolution model (cyan circles) taking into account the average observational errors on age and metallicity (following Eqs. (10) and (11)) computed for different age ranges. Also plotted are the observational data, colour coded as in Fig. 1. Upper left panel: stars older than 11 Gyr. Upper right panel: stars formed throughout the Galactic history. Lower left panel: stars younger than 8 Gyr. Lower right panel: stars younger than 4 Gyr.