We compare X-ray and caustic mass profiles for a sample of 16 massive galaxy clusters. We assume hydrostatic equilibrium in interpreting the X-ray data, and use large samples of cluster members with redshifts as a basis for applying the caustic technique. The hydrostatic and caustic masses agree to better than ˜20 per cent on average across the radial range covered by both techniques (˜[0.2-1.25]R500). The mass profiles were measured independently and do not assume a common functional form. Previous studies suggest that, at R500, the hydrostatic and caustic masses are biased low and high, respectively. We find that the ratio of hydrostatic to caustic mass at R500 is 1.20^{ 0.13}_{-0.11}; thus it is larger than 0.9 at ˜3s and the combination of under- and overestimation of the mass by these two techniques is ˜10 per cent at most. There is no indication of any dependence of the mass ratio on the X-ray morphology of the clusters, indicating that the hydrostatic masses are not strongly systematically affected by the dynamical state of the clusters. Overall, our results favour a small value of the so-called hydrostatic bias due to non-thermal pressure sources.
