The meta-analysis by Olivier and Creighton (2016) is the most comprehensive one. It is based on 40 studies from 1989 to 2016 and concludes that bicycle helmets reduce head injury by 51%, serious head injury by 69% and fatal head injury by 65%. With the help of meta-regression analysis, Olivier and Creighton (2016) show that the effects of bicycle helmets differ by type of injury, but that none of the following factors are likely to have affected the results: Setting for data collection, pro- vs. retrospective data collection, mandatory helmet legislation (yes vs. no), age category (adults vs. children), proportion of males, proportion of bicycle-motor vehicle collisions, time trend bias (publication year), and publication bias either.
The aim of the present study is to replicate the results from Olivier and Creighton (2016), based on a larger number of studies, and to investigate the effects of some additional potential confounding variables. The present meta-analysis includes 15 studies in addition to the 40 studies included in the meta-analysis by Olivier and Creighton (2016). In addition to most of the potential confounding variables investigated by Olivier and Creighton (2016), it investigates the effects of controlling for potential confounding variables in the individual studies, differences between hospital vs. emergency department (ED) or police reported data, and effects of the choice of comparison group on the estimated effects of bicycle helmets on cervical spine injuries. Publication bias has been tested by inspecting funnel plot diagrams for each type of injury, and by using the trim-and-fill (T&F) method developed by Duval and Tweedie (2000).