Posterior distribution for Y_tot and θ_s for simulated clusters with realistic CMB and noise levels (see text for details), and a) differing GNFW shape parameter values (γ,α,β) based on the REXCESS sample (Böhringer et al. 2007; Arnaud et al. 2010); and b) simulated with the universal values. In all cases the model used for recovering the parameters has the shape parameter values fixed to the universal values, and the joint two-dimensional prior on Y_tot and θ_s is used. Results for three different angular sizes are shown (from top to bottom, θ_s = 1.8,4.5 and 7.4); the input parameter values are marked with red triangles. The contours are at the 68% and 95% confidence boundaries.