1. An apparatus for performing verification tests on a system under verification representing at least part of the design of a data processing system, the apparatus implemented on a general purpose computer, said apparatus comprising: a transactor for connecting to an interface of the system under verification, and for generating signals for input to the system under verification via said interface during performance of said verification tests; profile storage for storing a profile providing a statistical representation of desired traffic flow at said interface, the statistical representation providing statistical information for a plurality of traffic attributes and identifying at least one dependency between traffic attributes in said plurality; and the transactor referencing said profile in order to determine the signals to be generated, such that the signals generated take account of said at least one dependency identified in the profile, wherein said profile employs a hierarchical structure to identify said at least one dependency within said statistical representation of desired traffic flow.