1. A method comprising: receiving at a data processing system: (i) a first identifier of a user, U 1 ; (ii) a first identifier of a tag, T 1 ; (iii) a number of times, M 1 , that the tag T 1 has been associated with the user U 1 ; (iv) a first identifier of a second user, U 2 ; and (v) a number of times, M 3 , that the tag T 1 has been associated with the second user U 2 ; and transmitting from the data processing system a second measure, M 2 , of a value of the user U 1 , based on a value of M 1 , wherein the second measure, M 2 , is based on the value of M 1 and M 3 .