Top: Bias of the UPP fitted to mock profiles with an intrinsic scatter of σ_int = 0.4. The colours indicate the results with different priors for M_500, i and δ, from case A to D, as detailed in the text. The solid lines show the mean bias for the 60 realisations, and the shaded areas cover the 16th to 84th percentiles. Bottom: Bias of the fitted individual cluster masses. For each cluster, we give the median bias and dispersion from the 60 realisations. The dashed lines indicate ±1σ.