18. The system of claim 16 , wherein identifying the discriminating attribute comprises clustering the plurality of users based on respective attributes of the users to identify the discriminating attributes that influence cluster membership.
18. The system of claim 16 , wherein identifying the discriminating attribute comprises clustering the plurality of users based on respective attributes of the users to identify the discriminating attributes that influence cluster membership. 19. The system of claim 18 , wherein clustering comprises: for different pairs of users of the plurality of users, calculating a respective distance between the users of the pair based on the respective attributes of the users in the pair; and clustering users based on the distances.