1. A fingerprinting method comprising, for each round in a series of rounds: providing to each receiver in a set of receivers a version of a source item of content, the source item of content corresponding to the round, wherein for the round there is a corresponding part of a fingerprint-code for the receiver, the part comprising one or more symbols, wherein the version provided to the receiver represents those one or more symbols; obtaining, from a suspect item of content one or more corresponding symbols as a corresponding part of a suspect-code; for each receiver in the set of receivers, updating a corresponding score that indicates a likelihood that the receiver is a colluding-receiver, wherein a colluding-receiver is a receiver that has been provided with a version of a source item of content that has been used to generate a suspect item of content, wherein said updating is based on the fingerprint-code for the receiver and the suspect-code; for each receiver in the set of receivers, if the score for the receiver exceeds a threshold, updating the set of receivers by removing the receiver from the set of receivers so that the receiver is not provided with a further version of a source item of content, wherein the threshold is set such that the probability that a receiver that is not a colluding-receiver has a score exceeding the threshold is at most a predetermined probability.