Posterior distributions for simulated clusters with realistic noise levels (see text for details), and varying GNFW shape parameter values based on the REXCESS sample (Böhringer et al. 2007; Arnaud et al. 2010). a) shows the two-dimensional θ_s and Y_tot posterior, and b) and c) show the one-dimensional posteriors for α and β, shifted to be centred on the appropriate true value. In all cases γ is fixed to the universal value, α has a truncated Gaussian prior based on the REXCESS sample, β is varied uniformly between 4.5 and 6.5, 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 arcmin); the input parameter values are marked with red triangles and lines.