Comparative accuracy of photometric redshift estimates as a function of the S/N of the sources. The plot shows, for a random sample of 20 000 sources from the PRIMER-COSMOS field, the S/N at the wavelengths corresponding to the photometric bands included in the catalogue, colour-coding each point with the standard deviation of the three photo-z estimates used to compute the median photo-z (see text for details). Objects with S/N <10 in the red bands (and/or S/N <1 in the blue bands) tend to have discordant photo-z estimates. See text for more details.