Reduced σ of photo-zs versus galaxy i′ absolute magnitude. From top to bottom: cluster galaxies inside a 500 kpc radius, inside a 1 Mpc radius, and field galaxies. Error bars for the absolute magnitudes are simply the second order momentum of the galaxy magnitude distribution in the selected magnitude bins ([−24; −23] , [−23; −22] , [−22; −21] , [−21; −20]). Error bars for the reduced σ are Poissonian error bars and are therefore directly proportional to the inverse of the number of galaxies inside the considered bin.