Relative mass bias δ_M (32) for magnified source number counts. Source galaxies are at z_s = 0.99 and lenses are chosen according to Table 1a with redshifts z_d = 0.41 (plus) and 0.62 (cross), for fixed limiting magnitude. The mass bias behaves roughly like the fractional shear difference (cf. Fig. 4). For local slopes α_s <  1, the underestimation of mass is stronger than for the shear profile, while for α_s >  1 the overestimate is similar.