Bias on the mean redshift (see Eq. (3)) estimated using the zPDF method and averaged over the 18 photometric noise realisations. Top and bottom panels: correspond to the mock DES/Euclid and LSST/Euclid catalogues, respectively. We note the differing scales in the y-axes of the two panels. Left panels: are obtained by summing the initial zPDF without any attempt at de-biasing. The other panels show the results of summing the zPDF after de-biasing, assuming (from left to right) a uniform, SOM, and COSMOS-like training sample. The yellow region represents the Euclid requirement of |Δ_⟨z⟩| ≤ 0.002 (1 + z). The red circles and black triangles in each panel correspond to the results estimated using photo-z and flat priors, respectively.