Figure-of-Merit (FoM) as a function of the fraction α of Euclid’s observed sky (f_sky). The blue curve represents the combination of a fraction α for the main probes (GC_sp and 3×2pt) and (1 – α) for the new observable O. The orange curve represents the combination of a fraction of α for GC_sp, together with the full 3×2pt analysis for the full survey and a contribution of (1 – α) for the new observable O. The green curve shows the FoM of the full survey with an additional external contribution from the new observable up to a factor or = 1.2. The dashed line represents the optimal FoM using the standard GC_sp and 3×2pt analysis. See the text for additional details on the different cases.