Accurate and precise mass measurements of clusters of galaxies are critical for constraining cosmology with cluster number counts from eROSITA. Much progress has been made in measuring cluster masses from X-rays (assuming hydrostatic equilibrium), weak gravitational lensing, and galaxy dynamics, but systematic differences between the methods still remain at the tens of percent level. We will present the largest comparison between cluster masses determined from X-ray hydrostatic analyses and galaxy dynamics using the caustic method. In a sample of 44 galaxy clusters we find evidence for a significant systematic difference with the hydrostatic masses being higher than the caustic masses by about 30% on average. We discuss possible reasons for this effect.
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