Sensitivity (relative errors with respect to the fiducial values) to the cosmological parameters, comparing ΛCDM cosmologies (solid colour bars) and w₀w_aCDM cosmologies (hatched colour bars), for models with (bottom panel) and without (top panel) varying N_eff. The different colours correspond to the sensitivity of different data combinations: WL+GC_ph+XC_ph+GC_sp (blue); Euclid +Planck (orange); and Euclid +CMB-S4+LiteBIRD (green). We note that for w_a we show the absolute errors, while for ΔN_eff we show the 95% upper limits. For the neutrino mass we always show the relative 1 σ uncertainty as computed from the posterior variance in order to facilitate the comparison between different data sets.