Another important source of information that can be used for query expansion is an application-specific thesaurus. These are surprisingly common since often an attempt will have been made to build them either manually or automatically
for a previous information system. Although they are often very incomplete,
the synonyms and related words they contain can make a significant difference to ranking effectiveness.
Having identified the various document features and other evidence, the next
task is to decide how to combine it to calculate a document score. An open source
search engine such as Galago makes this relatively easy since the combination and
weighting of evidence can be expressed in the query language and many variations
can be tested quickly. Other search engines do not have this degree of flexibility.