Ratio of the Y₅₀₀ Compton parameter to the normalised Y_X parameter. Left panel: variation as a function of redshift. The dotted line is the REXCESS prediction (Arnaud et al. 2010). The full line is the best fit power law and the grey shaded area indicates the  ± 1σ uncertainty. Clusters with normalised Y_X ≲ 5 × 10^-4arcmin² (green points) were excluded from the fit, to minimise Malmquist bias. Right panel: histogram of the ratio without and with low flux clusters.