B-spline decomposition of the HMF for SO haloes with ∆ = 200c at z = 0 in wCDM (left) and f(R)CDM (right) cosmologies. Top panel: measured HMF from the simulations (data points), with the corresponding errors bars (not always visible), and B-spline function fits (dashed lines). Each colour corresponds to a different training model. For visual clarity, only the models from the first slice of the quasi-PLHS are displayed (i.e. 16 models). The dashed vertical lines indicate the location of the B-spline knots. Middle panel: distribution of the fit residuals normalized by the noise in the simulation data. The dark and light shaded areas mark the 68.2% and 95.4% confidence intervals, respectively. The dashed lines indicate the ±1 and ±2σ levels. Bottom panel: distribution of the fit residuals normalized by the simulation data (i.e. the relative errors). The dark and light shaded areas mark the 68.2% and 95.4% confidence intervals, respectively. The shaded region corresponds to the ±1% level, along the y-axis the scaling is linear within the 1% region and logarithmic outside. The uncertainties on the B-spline fits are dominated by the simulation noise, except in the wCDM case at small masses, where they saturate at the ~0.5% level.