Rebutting helmets reduce head injuries by 85%
Key paragraphs from the article: http://www.thewashcycle.com/2013/02/bike-helmets-reduce-injuries-by-about-10-20.html
The best way to know the effectiveness of a drug or helmet is to get a
representative sample of the population, and then randomly assign them to
the test group or the control group. Such an experiment allows one to
reliably estimate effectiveness subject to a statistical margin of error.
But we don’t know who will be involved in a crash, and if we did, it would
be unethical to tell them whether to wear a helmet. Instead, researchers
collect data after the fact.
In 1989, Thompson et al. obtained data from Seattle hospitals for two
groups of cyclists who went to the hospital after a crash. Only 7% of the
first group wore a helmet, and they all had head injuries. But 24% of the
second group wore helmets, and none of them had head injuries. Assuming
that both groups were the same except for the type of injury they
experienced, these results imply that helmets reduced head injuries by
75%. Thompson et al. realized that the two groups were different, ran
regression analysis on the data to isolate the effects of helmets and
found that helmets were even more effective: 85%.
That study led the researchers to start saying two things that have almost
become mantras among many public safety advocates: “Helmets reduce head
injuries by 85%” and “The most important thing you can do to be safe on a
bike is wear a helmet.”
In the last 24 years, similar studies have found that helmets reduce head
injuries, but to a less extent than in the Seattle Study. A comprehensive
synthesis of all studies in 2001 estimated 53-63% effectiveness, but
because helmets increase neck injuries, the net effectiveness is 41--0%.
Studies in the last decade have estimated that helmets only prevent 20-40%
of potential head injuries, so the most recent synthesis of all studies
ever published finds the helmets reduce head injuries by 30--50% and total
injuries by 10-20%, when you include the increased neck injuries. But we
still hear the refrain “helmets reduce injuries up to 85%!”