Distribution of the 68% range of the photo-z solutions (Δz) as a function of the associated best-fit photo-z for the 436 GROND sources identified as galaxies (empty circles). Candidate members of XMMU J0338.7 + 0030 are reproduced with red (green) filled circles if they are within (outside) the bona fide cluster region; the three spectroscopic members among them are marked with blue filled circles (see Sect. 3.3.2). The dotted, short-dashed, long-dashed and solid lines represent values of Δz equal to 0.4, 0.2, 0.1 and 0.05 × (1 + z), respectively.