Modern interconnect structures may be regarded as a large number of basic units : pairs of Al alloy lines at two levels interconnected by vias. Presently as many as six levels of wiring may be used. Generally the vias present barriers to the transport of components of the alloy between levels. While it is recognized that the failure statistics of such structures are different from the conventional test structure, different values of activation energy for failure have been reported. Consequently, different interpretations may be made regarding mechanisms. Simulations are presented to demonstrate that differences between test structures can result in different measurements. It is shown that simple lines, passivated with native oxide, need to be longer than about seven times the critical length and lines terminate in pads at the anode end about four times this length, if measurements of both lifetime and activation energy are to be representative of long lines.