A good review of a medical intervention starts by explaining the population being studied. The Cochrane review of helmets for preventing head injuries in bicyclists explains that its population is the set of bicyclists who sustained an injury that was sufficiently major to make them go to the ER for treatment (and not sufficient to kill them before they could seek treatment).
Continue reading at At War With The Motorist for a discussion of the subtleties of bicycling that were missed in the best research we have on bicycle helmets.