Top: comparison between mass estimates obtained following the forwards method (M_for) and backwards method (M_back) for the 47 clusters of our sample. The lower panel shows the M_for/M_back ratio of individual clusters against M_back. The dashed line shows the one-to-one relation. The comparison is made at the outermost radius measured in the spectral analysis for each cluster. Middle: distribution of the mass ratios. Bottom: distribution of the relative errors.