14. A method for improving search and retrieval of resources in a collaborative tagging system, the method comprising: identifying using a computing system in communication with a plurality of computer accessible resources relationships between pairs of tags selected from a plurality of tags in the collaborative tagging system using sub-tag relationships between the pairs of tags based on co-occurrence of those pairs of tags at a given resource and associative semantics, the associated semantics comprising common association between tags in each given pair of tags by the plurality of users, by calculating a conditional probability for each pair of tags that if a first tag in a given pair of tags has been used to tag a given resource a second tag in the given pair of tags has also been used to tag that resource by determining a ratio, for each pair of tags, of a number of resources tagged by both first and second tags in a given pair to a number of resources tagged by the second tag in the given pair, the plurality of tags associated by a plurality of users with the plurality of resources; using the identified tag relationships based on tag co-occurrence statistics to create a hierarchy of tags comprising a directed acyclic graph having a plurality of nodes, each node comprising one of the plurality of tags, and a plurality of edges, each edge comprising one of the identified tag relationships; and using the created hierarchy to infer additional tags for resources automatically and to increase a number of tags associated with each resource in order to increase a total number of resources uncovered by a tag-based search of the plurality of resources.
18. The method of claim 14 , wherein the step of using the created hierarchy further comprises using the created hierarchy to maximize the recall of a tag cloud comprising a plurality of tags.