The original tests were a 10-pound block of wood dropped onto a helmeted head form. Now we drop the head form. The sensors that are used now are three-dimensional accelerometers, so we measure the acceleration experienced by the head in striking the ground. That measure of acceleration has been linked with skull fracture, and that has really been our measure over the many years.
As I said, we have virtually eliminated that type of injury. The only time a skull fracture happens really is if the person loses their helmet.