Marginalised posterior distributions of the best-fit scaling relation parameters between velocity dispersion and M_200c. The filled 2D contours show the 1-σ and 2-σ confidence levels of the posteriors after convolution with the uniform priors. The model is given by Eq. (17). The corresponding 1D parameter constraints are reported in Table 3. The top panel refers to direct relation between true velocity dispersion and halo mass, with black contours referring to the full sample, orange ones to clusters and groups detected in the optical mock, and blue ones with the addition of the X-ray detection. The bottom panel refers to the full model of the scaling relation accounting for the measured velocity dispersion.