Confounding between the child's restraint use and driverbehavior can bias restraint effectiveness estimates away from
the null if survivable crashes are more common in certain restraint
types. Analyzing only fatal crashes may introduce
selection bias toward the null because any protective effects
of a restraint type will underrepresent children in that
restraint. A marginal-structural-model-type estimator suggests
a 17% reduction in fatality risk for children aged 2
through 6 in child restraint systems relative to seat belts.
This reduction is estimated at 22% when severe misuse of
the restraint is excluded.