Empirical cumulative distribution function (ECDF) of index values, I₅, calculated for sample S₁. The dashed black line denotes the significance threshold of the sample (Table 2). The intercept between the significance threshold and ECDFs is equal to 1−F, where F is the fraction of sources YSOs with an index value greater than this (I₅ >  I_sig) for each class. As can be seen, the ECDFs of each class are distinct which indicates the differences in their clustering behaviours are significant.