The first building block to use galaxy clusters in astrophysics and cosmology is the accurate determination of their mass . Two of the most well regarded direct mass estimators are based on weak lensing ( WL ) determinations or X-ray analyses assuming hydrostatic equilibrium ( HE ) . By comparing these two mass measurements in samples of rich clusters , we determined the intrinsic scatters , \sigma _ { \mathrm { WL } } \sim 15 per cent for WL masses and \sigma _ { \mathrm { HE } } \sim 25 per cent for HE masses . The certain assessment of the bias is hampered by differences as large as \sim 40 per cent in either WL or HE mass estimates reported by different groups . If the intrinsic scatter in the mass estimate is not considered , the slope of any scaling relation ‘ observable–mass ’ is biased towards shallower values , whereas the intrinsic scatter of the scaling is over-estimated .