A fingerprinting method. 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. For the round there is a corresponding part of a fingerprint-code for the receiver, the part includes one or more symbols. The version provided to the receiver represents those one or more symbols. One or more corresponding symbols are obtained from a suspect item as a corresponding part of a suspect-code. For each receiver in the set of receivers, a corresponding score that indicates a likelihood that the receiver is a colluding-receiver is updated.