One such article of note, was from 2012, Nonuse of bicycle helmets and risk of fatal head injury: a proportional mortality, case–control study, by Navindra Persaud et al., which claims that you're 2.5 times more likely to have a fatal head injury as a result of not wearing a helmet, with an odds-ratio increasing to 3.1 when "adjusted for age and sex".
Results: Not wearing a helmet while cycling was associated with an increased risk of dying as a result of sustaining a head injury (adjusted odds ratio [OR] 3.1, 95% confidence interval [CI] 1.3–7.3). We saw the same relationship when we excluded people younger than 18 years from the analysis (adjusted OR 3.5, 95% CI 1.4–8.5) and when we used a more stringent case definition (i.e., only a head injury with no other substantial injuries; adjusted OR 3.6, 95% CI 1.2–10.2). Interpretation: Not wearing a helmetThis study was note worthy as it was the first research into fatal head injuries and the efficacy of helmets. And as always, when ever an article like this is published, within microseconds of the abstract being made public, the media are quick to promote it. So how was this (mis-)reported by the papers?
The suggestion by the abstract and the media is you're three times more likely to die without a helmet....
This Is Massively Misleading
Lets look at the results, carefully, to see why it's misleading.
There was 129 cycling deaths studied by coroners in Ontario from 2006-2010. Out of these deaths, 34 cyclists wear wearing helmets, and the rest, 95 cyclists were not wearing a helmet.
In studies like this, in order to estimate an odds ratio, a control group is required. Autopsies were conducted to determine the cause of death, and as I understand it, the pertinent groups were
- death due entirely to other injuries, without a fatal head injury
- death entirely and solely due to a fatal head injury, without other fatal injuries
- a fatal head injury with other injuries
The first of the above list comprised the control group. Helmet wearing for the controls was around 36%, which they claim to be consistent with a 2009 survey.
Their results are in table 3, and are thus:
|Case definition||Fraction not wearing a helmet||OR (95% CI)||Adjusted*|
OR (95% CI)
|Head injury as cause of death with other injuries||58/71||37/58||2.5 (1.2–5.7)||3.1 (1.3–7.3)|
|Head injury as cause of death with no other injuries||38/43||57/86||3.9 (1.4–10.9)||3.6 (1.2–10.2)|
Note: CI = confidence interval, OR = odds ratio.
*Adjusted for age and sex.
THESE RESULTS ARE NOT ODDS-RATIO FOR DYING AS A RESULT OF NOT WEARING A HELMET.
Take row 2 for example, it means out of the group that died solely as a result of a fatal head injury, the odds-ratio was 3.6. But note the controls! 86 people out of the 129 died as a result of other injuries. They would have died anyway, helmet or no helmet.
How do calculate the actual odds of dying in without a helmet from the above data for the cyclists in this study group?
(95/34) x (0.36/ (1 - 0.36)) = 1.57
Where 0.36 was their measured helmet wearing rate in the controls, and as above, 95 cyclists were not wearing a helmet, and 34 were.
Further caveats, this odds ratio is pertinent to crashes of this severity, and is subject to the controls being correctly identified. A small bias in mis-classifying the controls, has a drastic effect on the supposed odds-ratio.
So the reported odds ratio in the media and the wording in the abstract results were utterly wrong, and completely misleading.