  Scarcity of labeled data is a common problem in supervised classification,
since hand-labeling can be time consuming, expensive or hard to label; on the
other hand, large amounts of unlabeled information can be found. The problem of
scarcity of labeled data is even more notorious in hierarchical classification,
because the data of a node is split among its children, which results in few
instances associated to the deepest nodes of the hierarchy. In this work it is
proposed the semi-supervised hierarchical multi-label classifier based on local
information (SSHMC-BLI) which can be trained with labeled and unlabeled data to
perform hierarchical classification tasks. The method can be applied to any
type of hierarchical problem, here we focus on the most difficult case:
hierarchies of DAG type, where the instances can be associated to multiple
paths of labels which can finish in an internal node. SSHMC-BLI builds
pseudo-labels for each unlabeled instance from the paths of labels of its
labeled neighbors, while it considers whether the unlabeled instance is similar
to its neighbors. Experiments on 12 challenging datasets from functional
genomics show that making use of unlabeled along with labeled data can help to
improve the performance of a supervised hierarchical classifier trained only on
labeled data, even with statistical significance.
