1. An information retrieval system comprising: an input arranged to receive user query terms; a processor arranged to generate a plurality of lists of documents using the query terms and by accessing a document index; a prediction engine arranged to predict, for each of the lists of documents, implicit relevance information and explicit relevance information; a utility engine arranged to calculate a utility score for each of the lists of documents using the predicted relevance information and a stored utility function; the utility engine also being arranged to select and store one of the lists ( 105 ) of documents on the basis of the utility scores.