Fractional histograms of the -log  Kolmogorov-Smirnov probability for three categories of objects. The solid line represents the REFLEX II clusters as in Fig. 4. We truncate the values so that the last bin contains the remaining counts for the whole sample, which amounts to 5%. The dotted lines mark 1542 non-cluster sources, and the red dashed lines are for the 22 clusters. The non-cluster sources are strongly concentrated at small extents.