From a 2006 study [1]:
Using only data from FARS to assess child restraint system effectiveness in reducing risk for death in passenger vehicle crashes implicitly assumes that surviving children in fatal accidents have restraint practices that are similar to those of children in other serious crashes that do not result in fatalities. If child restraint systems are highly effective in reducing risk for death relative to seat belts, children in crashes who survived because they were restrained in a child restraint system rather than a seat belt will not be included in the FARS database unless someone else in the crash died. This will lead to a potential underrepresentation of child restraint system use among surviving children and a consequent underestimation of the effectiveness of child restraint systems in reducing risk for fatal injury if only FARS data are used.
[1] Effectiveness of Child Safety Seats vs Seat Belts in Reducing Risk for Death in Children in Passenger Vehicle Crashes http://archpedi.jamanetwork.com/article.aspx?articleid=20506...