By Nicholas Kevlahan
Published October 16, 2012
Several recent media articles have reported on a new study, based on 2006-2010 data from the office of the Ontario Coroner, finding that cyclists who die of a head injury in a crash or collision were three times less likely to be wearing a helmet than cyclists who died of other injuries. The Globe and Mail reports:
Researchers looked at 129 accidental cycling deaths in Ontario between 2006 and 2010, using data from the Office of the Chief Coroner.
They found cyclists who didn't wear a helmet were three times more likely to die of a fatal head injury than those who wore head protection while riding.
The study, titled Nonuse of bicycle helmets and risk of fatal head injury: a proportional mortality, case-control study [PDF], was published in the October 15, 2012 issue of the Canadian Medical Association Journal.
The study notes that the researchers used an adjusted odds ratio to analyze the correlation between helmet use and reason for death.
An odds ratio is not a measure of relative risk but rather of the strength of the association between two values being measured.
As Wikipedia warns:
Odds ratios have often been confused with relative risk in medical literature. For non-statisticians, the odds ratio is a difficult concept to comprehend, and it gives a more impressive figure for the effect. However, most authors consider that the relative risk is readily understood. In one study, members of a national disease foundation were actually 3.5 times more likely than nonmembers to have heard of a common treatment for that disease - but the odds ratio was 24 and the paper stated that members were 'more than 20-fold more likely to have heard of' the treatment.A study of papers published in two journals reported that 26% of the articles that used an odds ratio interpreted it as a risk ratio.
This may reflect the simple process of uncomprehending authors choosing the most impressive-looking and publishable figure. But its use may in some cases be deliberately deceptive. It has been suggested that the odds ratio should only be presented as a measure of effect size when the risk ratio can not be estimated directly.
In the case of the bicycle helmet study, the reporting makes it sound like a cyclist not wearing a helmet is three times as likely to die as a cyclist wearing a helmet, which is incorrect.
In fact, the authors themselves seem to confuse odds ratio and relative risk, as the title is Nonuse of bicycle helmets and risk of fatal head injury: a proportional mortality, case-control study and the introduction states, "We sought to determine whether cycling without a helmet was associated with an increased risk of sustaining a fatal head injury."
The study results do suggest that bicycle helmets provide some protection in the case of collision with motor vehicles, but the use of odds ratios (standard for this sort of case control study) rather than relative risk may exaggerate the benefit significantly.
The actual data on the effects of mandatory helmet laws do not support the principal authors' claim that their results strongly support such legislation. Specifically, helmet laws tend to deter cycling, and lower rates of cycling are associated with a higher risk of collision for each cyclist.
This is a case where an evidence-based decision requires looking at everything affecting road safety, including avoiding accidents, and international best practice in countries such as the Netherlands and Denmark, neither of which emphasize helmets as the most important aspect of safe cycling.