Tests to identify possible sources of systematic error. The left panel shows the three runs used in this paper and how the force resolution affects concentration measurements. There is excellent agreement between the GS and HACC runs, even though they were run using two completely different codes with different settings. The lower-resolution G run (resolution =35  h ⁻¹ kpc) is systematically lower by about 5% compared to the HACC run (resolution =7  h ⁻¹ kpc). We also study the possible effect of a misestimate of the halo center for the G run which can lead to a further reduction of concentration values. The right panel studies two other systematics issues: (1) the profile range used for fitting and (2) effect on concentration measurements when halos with large radial infall velocity are removed from the sample (solid curve). See the text for further discussion.