R − v_los diagram (top panels) and mass profile (bottom panels) of two simulated clusters in the low-mass bin. Left (right) column: cluster at z = 0.12 (z = 0.19). In the bottom panels, the red (blue) solid curves show the caustic (real) mass profile, whereas the red (blue) dot-dashed curves show the NFW fits to the caustic (real) mass profile. In these two examples, the two NFW fits are indistinguishable. The shaded areas show the 50% confidence level of the caustic location and of the caustic mass profile according to the caustic technique recipe. In the R − v_los diagrams the shaded areas are present, but very thin.