1 ) A method for clustering a plurality of content items, the method comprising, in a processing system: a) determining intervals between at least some of the content items; b) selecting a seed pair of content items using the determined intervals; c) assigning the seed pair to a current cluster; d) determining an interval between one of the content items in the current cluster and a neighbouring one of the content items; e) comparing the interval to a predetermined criterion; and, f) in response to a successful comparison, assigning the content item to the cluster.