1. A method for determining if a first and second document stored in a digital format in a data processing system are similar by comparing sparse representations of the two documents, the method comprising the steps of: breaking the first and second documents into chunks of data of predefined sizes; selecting a subset of all chunks as representative of the data in the document; determining a set of coefficients that represent selected chunks; combining sets of coefficients into coefficient clusters, a coefficient cluster containing coefficients which are similar according to a predetermined similarity metric; and evaluating a degree of similarity between documents by counting clusters into which chunks from both documents fall.