Dispersion of the vertical velocity as a function of metallicity (top panels), [α/Fe] (middle panels), and stellar age (bottom panels) for the three stellar populations: thin disc (orange), hαmp thick (blue), and hαmr thick disc (magenta). The whole thick disc (i.e. hαmp + hαmr thick discs) is also shown with the green dashed line. σ_Z are computed from BGM (left panels) and from observations using the APOKASC sample (right panels). BGM predictions for the thin disc and the ‘whole’ thick disc are overplotted on the right panels with grey solid and dashed lines, respectively.