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Assessing proximal risks for acute traumatic events: the case-crossover design

When assessing risk factors for injuries, one can focus on trait-like factors (e.g., static or time-independent characteristics of the individual, agent or environment) or on state-like factors (e.g., changing or time-dependent behaviours or circumstances) (Kuhn & Damashek, 2015). Statelike factors are measured over time, and ideally, proximal or immediately prior to the occurrence of the event of interest (e.g., an injury). While trait-like factors provide valuable information, state-like factors may help explain what specific circumstances or behaviours immediately preceded the injury event. The proximal nature of the factor is viewed by some as the probable ‘immediate cause’ of the event. In the injury research field, events are acute as well as traumatic, and understanding the role of proximal risks is key. For example, we may be interested in the proximal risks for a car crash what factors in the environment, the vehicle or the person, were present in the time period immediately preceding the crash, and whether they were present or different in a control time period in which there was no crash event. Research on proximal risks requires the use of appropriate study designs and analytic methods. This Accidental Note reviews the case-crossover design (Maclure, 1991) and the appropriate statistical analytic methods.