Assessment on the robustness of the secondary bias measurements against potential intra-bin mass dependences of group bias. Each line corresponds to the relative bias of the upper or lower subsets computed assuming the Sheth et al. (2001) halo bias model, where the mass assigned to each subsample corresponds to the mean mass. For reference, the shaded grey and green regions show the maximum intra-bin bias difference, obtained from the edges of the group mass bin; here both the Sheth et al. (2001) model and the Tinker et al. (2010) analytical fit are employed.