As concussion became the sports injury “issue” of the 1990s, numerous researchers began projects regarding concussions. It became apparent from the findings of the earlier projects that a variety of different methods of identifying a concussion exist. Some clinicians described a concussion as a loss of consciousness. Others identified a concussion only if memory problems were associated with the injury. Still other clinicians considered a very minor impact to the head, often called a “ding,” to be a concussion. The confusion over the definition created problems for multicenter research programs. To be comfortable with the consistency of data coming from different locations, researchers ' projects began to identify mild traumatic brain injury