P_1.4 GHz − M₅₀₀ diagram for the clusters of the statistical sample (left) and extended sample (right). Black filled circles are radio halos, arrows are upper limits, and green empty circles are USSRHs or candidate USSRHs. The black line and grey shadowed region show the best-fit relations (using the BCES Y|X method) and the 95% confidence region to radio halos only. For a clear comparison, also shown is the best-fit relation (right panel, dashed line) obtained for the statistical sample (left panel).