“…Hierarchical classification typically employs a label hierarchy in the form of a tree [8,6,5,22,1,18] or directed acyclic graph [7,12] that explicitly injects prior knowledge of the label relationships into the model. The relationship between the labels can be either 'IS-A' or not depending on the associated classification problem [1,8,7,6,5,22,18,3,2]. One could either define the label hierarchy manually with domain knowledge [1,12,18] or derive the hierarchy automatically [8,6,5,7,22] from a well established semantic lexical database, such as WordNet [16].…”