Method and system for generating grammar rules

An information retrieval system including a natural language parser (3) for parsing documents of a document space (1) to identify key terms of each document based on linguistic structure, and for parsing a search query to determine the search term, a feature extractor (4) for determining an importance score for terms of the document space based on distribution of the terms in the document space, an index term generator (5) for generating index terms using the key terms identified by the parser and the extractor and having an importance score above a threshold level, and a query clarifier (16) for selecting from the index terms, on the basis of the search term, index terms for selecting a document from the document space. A speech recognition engine (12) generates the query, and a bi-gram language module (6) generates grammar rules for the speech recognition engine using the index terms.