Performances of the richness proxy (i.e., when the true r₂₀₀ is unknown). Left-hand panel: mass-predicted from richness vs. true (caustic) mass. The solid line is the one-to-one relation (not a fit to the data). The dotted lines show the above line plus or minus the data scatter (0.16 dex). The open point marks the outlier Abell 1068 cluster. Right-hand panel: mass residuals (predicted minus true) with superposed a Gaussian with σ = 0.16 dex.