Selection of controls in injury case-control studies

PURPOSE: To assist in elucidating principles underlying the design of injury case-control studies.APPROACH: We begin by defining "event" as the sequence of circumstances that place a person at risk of injury (e.g. bicycle crash) and "injury given the event" as the resultant physical damage (e.g. head injury in bicycle crash). We then identify two broad classes of research question: 1) Studies of risk factors for the event, and, 2) Studies of risk factors for injury given the event. The study base for the first type of research question is all persons at risk of the event, while the study base for the second type is all persons who experience the event, and are therefore at risk of injury. It follows that in studies of risk factors for injury given the event, the controls should be a sample of all persons who experience the event. For example, in a study of bicycle helmets and head injuries, a suitable case group would be cyclists treated for head injury following a bicycle crash. The appropriate control group is drawn from the base population of all cyclists who crashed, including those who had no injuries. The control group may be restricted to cyclists who crashed and sought treatment for non-head injury under the assumption that the exposure distribution (prevalence of cycle helmet use) in the crash/no injury group is identical to the exposure distribution in the crash/non-head injury group.CONCLUSIONS: It is over ten years since innovative researchers in Seattle first applied the case-control design to the problem of bicycle crashes. Since then, successive bicycle injury studies at other centers have largely failed to extend and refine the Seattle methodology. A more critical approach to the design of case-control studies is required if we are to continue to advance the field of injury epidemiology.