CAR OCCUPANT COULD BE killed if struck by another occupant who was catapulted forward, backward, or sideways in a crash. 1-3 We used a matched cohort study design to estimate the association between the death of a car occupant and the restraint use of another occupant in the same car. METHODS Matched Cohort Study Design Persons entered this cohort study when they crashed in a car. The study outcome was death within 30 days of the crash. We wished to adjust for potential confounding factors, such as crash speed and the ability of the vehicle to protect occupants. It is difficult to measure these variables accurately, and estimates of speed were often missing in the data we used. We therefore restricted our study to pairs of occupants who were in the same car when they crashed; these pairs were therefore matched in regard to all vehicle and crash characteristics, such as vehicle type, speed, crash location, and crash severity. A matched-pair cohort design eliminates confounding by all matched variables. 4-8 In addition, risk ratios (RRs) for death may be estimated for all pairs, including those in which both survived, using information only from pairs with at least 1 member who died. 4-8