Distribution of clusters over the AD test α_AD values for the SPIDERS and SCGF samples. The SPIDERS sample is marked with grey and dashed edges and SCGF with light blue and solid edges. The threshold limit for structure at α_AD = 0.05 is marked with a dash-dotted black line, and the threshold for possible substructure α_AD = 0.15 is marked with a dashed magenta line. The heights of the bars are calculated as the number of clusters per bin divided by the total number of clusters in the sample.