Learning in intelligent information retrieval . Lewis, D. 1991. Learning in intelligent information retrieval . These relations may be subsumption, associative, transitive, or unconstrained. Thesauruses can be either automatically created for which there exist a wide range of techniques, or they can be hand crafted.
Pronouns also cause a problem in cases where the number of references to a keyword are counted.
David D. Lewis. Learning in intelligent information retrieval . Proceedings of the Eighth International Workshop on Machine Learning. Rather than requiring that users anticipate exactly the words and combinations of words that will appear in documents of interest, statistical IR approaches let users simply list words that are likely to appear in relevant documents. David D. Lewis.