Bias on the mean redshift (see Eq. (3)) averaged over the 18 photometric noise realisations. The mean redshifts are measured using the direct calibration approach. The tomographic bins are defined using the DES/Euclid and LSST/Euclid photo-z in the top and bottom panels, respectively. The yellow region represents the Euclid requirement at 0.002 (1 + z) for the mean redshift accuracy, and the dashed blue lines correspond to a bias of 0.005 (1 + z). The symbols represent the results obtained with different training samples: (a) uniformly selecting 1000 galaxies per tomographic bin (black circles); (b) selecting two galaxies per cell in the SOM (red squares); and (c) selecting a sample that mimics real spectroscopic survey compilations in the COSMOS field (green triangles).