1. A method for determining the semantic similarity of words in a plurality of words selected from a set of one or more documents, for use in the retrieval of information in an information system, comprising the steps of: (i) for each word of said plurality of words: (a) identifying, in documents of said set of one or more documents, word sequences comprising the word and a predetermined number of other words; (b) calculating a relative frequency of occurrence for each distinct word sequence among word sequences containing the word; and (c) generating a fuzzy set comprising, for word sequences containing the word, corresponding fuzzy membership values calculated from the relative frequencies determined at step (b); and (ii) calculating and storing, for each pair of words of said plurality of words, using respective fuzzy sets generated at step (i), a probability that the first word of the pair is semantically suitable as a replacement for the second word of the pair.