One approach to cluster-based retrieval is to retrieve one or more
clusters in their entirety in response to a query [8, 22]. The task for
the retrieval system is to match the query against clusters of
documents instead of individual documents, and rank clusters
based on their similarity to the query. Any document from a
cluster that is ranked higher is considered more likely to be
relevant than any document from a cluster ranked lower on the list.
This is in contrast to most other cluster search methods that use
clusters primarily as a tool to identify a subset of documents that
are likely to be relevant, so that at the time of retrieval, only those
documents will be matched to the query. This approach has
been the most common for cluster-based retrieval.