  Many social Web sites allow users to publish content and annotate with
descriptive metadata. In addition to flat tags, some social Web sites have
recently began to allow users to organize their content and metadata
hierarchically. The social photosharing site Flickr, for example, allows users
to group related photos in sets, and related sets in collections. The social
bookmarking site Del.icio.us similarly lets users group related tags into
bundles. Although the sites themselves don't impose any constraints on how
these hierarchies are used, individuals generally use them to capture
relationships between concepts, most commonly the broader/narrower relations.
Collective annotation of content with hierarchical relations may lead to an
emergent classification system, called a folksonomy. While some researchers
have explored using tags as evidence for learning folksonomies, we believe that
hierarchical relations described above offer a high-quality source of evidence
for this task.
  We propose a simple approach to aggregate shallow hierarchies created by many
distinct Flickr users into a common folksonomy. Our approach uses statistics to
determine if a particular relation should be retained or discarded. The
relations are then woven together into larger hierarchies. Although we have not
carried out a detailed quantitative evaluation of the approach, it looks very
promising since it generates very reasonable, non-trivial hierarchies.
