Comparing a hierarchical and a flat partition of elements in three different contexts. In these dendrograms, the grey dots indicate the inner nodes that generate clusters to be tested against a dichotomic non-hierarchical classification, which establishes that elements belong (white dots) or not (black dots) to a particular class. In Figure 1A, it is shown the particular case in which two dichotomic flat partitions are compared, but one of them is represented, on the left, as a hierarchical structure. In Figure 1B, a second particular case: provided that the elements in the tree are ordered according to a score, this is equivalent to determining whether the top scorers in an ordered list are enriched for a particular feature. Figure 1C shows a more complex topology, illustrating the general case of comparing a hierarchical and a non-hierarchical classification.