11. A method as claimed in claim 1 which further comprises identifying clusters having a number of items below a specified threshold and absorbing those identified clusters into a single cluster.
11. A method as claimed in claim 1 which further comprises identifying clusters having a number of items below a specified threshold and absorbing those identified clusters into a single cluster. 13. A method as claimed in claim 11 wherein the items are advertisements and the features are keywords to which the advertisements are subscribed.