1σ MCMC confidence contours of Ω_M − b_eff obtained with different assumptions: standard analysis, as described in Sect. 4 – black; considering C1+C2 XXL clusters, instead of C1 only – red; assuming Planck15 as reference cosmology, instead of WMAP9 – green; with jackknife covariance, instead of bootstrap – blue; considering the angular mask to construct the random catalogue, instead of the technique described in Sect. 3.3 – magenta; considering the fitting scale range 1 < r[h⁻¹ Mpc] < 50, instead of 10 < r [h⁻¹ Mpc] < 40 – yellow; doubling thestatistical mass errors – brown; reducing the masses by 20% – cyan; assuming the Sheth et al. (2001) bias model to compute the effective bias of the sample – orange.