Observed velocity dispersion (σ_v, obs) of groups determined using their spec-z members within r_200c versus the velocity dispersion predicted by the virial theorem (Eq. (3), σ_v, VT), in COSMOS based on L_x- halo mass scaling relation in observations (upper panel) and using the virial mass in the HORIZON-AGN simulation (lower panel). Groups with N ≥ 5 are plotted with filled blue circles and red squares. Groups with N ≥ 10 spectroscopic members are shown with open green circles. The solid and dashed black lines show the 1:1 relation and ±15% intervals. The filled red squares and blue circles represent groups in S-I and S-II to S-IV respectively.