Shifts in units of standard deviations for Ω_m, 0 (left panel), w₀ (middle panel), and w_a (right panel) due to an incorrect assumption on the IA amplitude. Results are shown for the optimistic case with WL+GC_ph (orange) vs. WL+GC_ph+XC (green). Solid (dashed) lines represent positive (negative) shifts with respect to the fiducial. The vertical grey line shows the case in which we assume no contribution from IA (𝒜_IA = 0). The grey-shaded region denotes the shifts larger than 3σ, for which the Gaussian approximation breaks down and the corresponding shifts should be interpreted with caution (see the text for details).