Left: distribution of mean redshift as a function of SOM cell for the fiducial MICE2 simulation. Right: distribution of the difference in SOM cell mean redshifts for the full photometric sample compared to the first tomographic bin. The figure demonstrates that the N(z) of the high-z SOM cells in the lowest tomographic bin is systematically different to those cells in the full sample. This effect creates the bias observed in the SOM direct calibration.