But no matter what you control for in the FARS data, the results don't change. In recent crashes and old ones, in big vehicles and small, in one-car crashes and multiple-vehicle crashes, there is no evidence that car seats do a better job than seat belts in saving the lives of children older than 2. (In certain kinds of crashes -- rear-enders, for instance -- car seats actually perform worse.)There is more info on their blog.
One conclusion that I draw is to not trust the medical literature on subjects like this. Pediatricians and other medicos often give pseudoscientific arguments about things like this, and they should really stick to medicine.