11. The method of claim 9 further comprising: assigning a weight to each of the remaining ones of the candidate expansion concepts based on a frequency in which the remaining candidate expansion concept co-occurs with the original query concept in a sample corpus.
9. A query expansion method in a system having a knowledge source storing a plurality of concepts formed from a predefined vocabulary set, the knowledge source maintaining a plurality of relationship links wherein each relationship link defines a relationship between a first grouping of the concepts and a second grouping of the concepts, the method comprising: generating, under control of one or more computers indexing concepts for a plurality of documents configured to be searched based on a search request, wherein the generating of the indexing concepts includes: maintaining under control of the one or more computers, a plurality of data structures mapping each of the plurality of concepts in the knowledge source to all words appearing in the concept; receiving, under control of the one or more computers, a particular one of the free-text documents; identifying based on the plurality of data structures, one or more of the plurality of concepts in the knowledge source mapped to a set of words appearing in the particular one of the free-text documents; and returning, under control of the one or more computers, the one or more identified concepts as candidates index concepts for the particular one of the free-text documents; receiving, under control of the one or more computers, an input query having an original query concept and at least one scenario identifier; generating, under control of the one or more computers, a list of candidate query expansion concepts based on the original query concept; filtering, under control of the one or more computers, the list of candidate expansion concepts based on the scenario identifier, wherein the filtering includes removing one or more of the candidate expansion concepts from the list if the one or more candidate expansion concepts are not included in one or more first particular groupings of the concepts in the knowledge source that have a specific relationship link with a second particular grouping of the concepts containing the original query concept, the specific relationship link being identified by the at least one scenario identifier; and generating, under control of the one or more computers, an expanded input query including the original query concept and remaining ones of the candidate expansion concepts.