1. A method of determining cluster attractors for a plurality of documents, each document comprising at least one term, the method comprising: calculating, in respect of each term, a probability distribution indicative of the frequency of occurrence of the, or each, other term that co-occurs with said term in at least one of said documents; calculating, in respect of each term, the entropy of the respective probability distribution; selecting at least one of said probability distributions as a cluster attractor depending on the respective entropy value.