Halfrauds
From ChapmanCentral
Halfords claims:
- Research shows that wearing a cycle helmet reduces the risk of head and brain injury by 88% and upper and mid face injury by 65%
The claim is false and misleading. The figure 88% has only one source, a very early study which is widely criticised for methodological issues. This study was written by a group that was already committed to helmet compulsion but lacked any evidential basis to support it. The group has a deep ideological commitment to helmets and has written outspoken articles in several journals on the subject. Importantly, in In 1996, the authors adjusted their own assessment of the reduction in head injury by helmets to 69%, but the original figure is still the one that is widely quoted by the ill-informed.
The source paragraph says:
- Seven percent of the case patients were wearing helmets at the time of their head injuries, as compared with 24 percent of the emergency room controls and 23 percent of the second control group. Of the 99 cyclists with serious brain injury only 4 percent wore helmets. In regression analyses to control for age, sex, income, education, cycling experience, and the severity of the accident, we found that riders with helmets had an 85 percent reduction in their risk of head injury (odds ratio, 0.15; 95 percent confidence interval, 0.07 to 0.29) and an 88 percent reduction in their risk of brain injury (odds ratio, 0.12; 95 percent confidence interval, 0.04 to 0.40).
The body of the report shows that these are a form of statistic known as an odds ratio - formally, this means that the figure does not predict a protective capability, it notes the ratio of probability of injury between two populations. For example, suppose that in a sample of 100 men, 90 have drunk wine in the previous week, while in a sample of 100 women only 20 have drunk wine in the same period. The odds of a man drinking wine are 90 to 10, or 9:1, while the odds of a woman drinking wine are only 20 to 80, or 1:4 = 0.25:1. Now, 9/0.25 = 36, so the odds ratio is 36, showing that men are much more likely to drink wine than women.
The paper itself does not support the assertion that helmets reduce the risk of brain injury by 88%. Analysis of the same source data has shown that helmets "reduce the risk" of broken legs, being Hispanic, rising alone rather than in a group, riding on the road rather than on a bike path and so on. It is, in other words, arrived at by comparing two completely different populations of cyclists, and ascribing 100% of the difference in injury profile, to a single observed difference in behaviour.
This paper has been severely criticised by a considerable number of people, both within and outside of the medical profession. At root, its methodology is seen as seriously flawed. This paper used two control groups as comparison for the head-injured case group. Group 1 comprised cyclists seeking emergency room treatment who did not suffer head injuries. There is no reason to suspect that this group differed from the case group in typical membership. Helmet wearing rates in this group were 5.9% for children under 15 and 39.4% for older cyclists. Group 2 were from families that were members of a single large Seattle healthcare organisation, who filled out a survey form on cycle accidents. On average, these families had higher income and educational achievements than the Seattle population at large. The group was dominated by children under 15 (86%) – adults were too few to be significant. To be included, all members of the group had to do was fall off their bikes during the year. They didn't have to visit hospital and, indeed, only 12% sought medical care for any injuries sustained. The helmet wearing rate for children under 15 in this group was 21.1%.
It so happens that a third control group is also available as a result of a concurrent study in Seattle1 in May 1987 (of which Rivara was also an author). This observed children under 15 riding around the town and recorded a helmet wearing rate for these children of only 3.1%. Moreover, children wearing helmets were much more often white than black or other races, and riding in parks or on cycle paths than on city streets. Clearly the population control (Group 2) was nothing like the same as Group 3 in either membership or helmet wearing rates. If Group 2 is considered to represent child cyclists in Seattle, then it may be concluded that helmets prevent 85% of head injuries. However, if the children observed riding around Seattle (Group 3) are considered more typical of child cyclists, then the conclusion would be that helmets have no significant benefit because the helmet wearing rate of head injured child cyclists (2.1%) was well within sampling error of the rate seen on the streets.
Although 5.9% of children under 15 in Group 1 wore a helmet, there were only 12 of these as an absolute number and only 3 helmeted children under 15 in the case group. The numbers wearing helmets are much too small for a valid comparison of whether this control differs significantly from the street study Group 3 in wearing rates. That being so, no conclusion can be reached based on this group about the effectiveness of helmets in reducing head injury.
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Sampling bias
The study is not randomised. The people included in both case and control groups are self-selecting samples, in that they choose whether to wear a helmet or not, how they cycle, their attitudes to risk, and many other variables that are therefore beyond the control of the researchers.
Types of injury
Of the 235 head injuries studied, 3 (1.2%) resulted in death and 6 (2.5%) were unconscious for more than 24 hours. Most of the other injuries were minor cuts and scratches.
The study does not distinguish facial injuries from other head injuries, although helmets would not prevent the former. 46% of head injuries were to the forehead only.
If facial injuries are excluded, the 85% reduction in head injuries comes down to 61%, but the number of cyclists wearing helmets is too small for this to be statistically significant2. In further re-working of Thompson's data, McDermott found that only 40% of head injuries would be reduced using approved helmets, though injury rates increased for the neck, extremities and pelvic region.
Crucially, the study did not include a single case of a helmeted cyclist in collision with a motor vehicle, yet the authors and others have promulgated the results as applicable to all types of cyclist crash.
Authors' acknowledgement of limitations
The authors acknowledged two sources of uncertainty: statistical error due to the fairly small sample, and bias in the sample: "We cannot completely rule out the possibility that more cautious cyclists may have chosen to wear helmets and also had less severe accidents".
In 1996, the authors adjusted their own assessment of the reduction in head injury by helmets to 69%, but the original figure is still the one that is widely quoted.
Real-world figures
Most helmet studies are of the "case-control" variety, comparing injured and control populations. These are all vulnerable to the sampling biases discussed above. Real-world data, form whole cyclist populations, will be more reliable as it will not have these errors. There are two countries where helmet laws have been passed, leading to very substantial changes in helmet wearing over a short period: New Zealand and Australia. Both have been extensively studied. In Australia, cyclist head injuries were found to decline by slightly less, in proportion, than the reduction in cycling due to the deterrent effect of the law - in other words, a population where roughly 90% of cyclists wore helmets, suffered a slightly higher injury rate in proportion than a population where around half of cyclists wore them. In New Zealand, the effect was similar: no measurable reduction in head injury rates as a result of an increase in around two years from around 40% to over 90% wearing rates.
There is another study which is not often quoted, by the United States consumer protection body CPSC. This study, to my knowledge the largest ever conducted, assessed all recorded cyclist injuries in the United States over a period of some years and concluded that "bicycle-related fatalities are positively and significantly associated with increased helmet use" - in other words, the helemt wearers were more likely to die than the non-wearers.
The only thing that can be conclusively stated about bicycle helmets is that nobody knows whether, and by how much, they might prevent injuries in an y given crash. To state the largest, oldest and most discredited figure as if it were still 1995, the Australian and New Zealand analysis unpublished, and the flaws in the research not noted, seems suspiciously close to fraudulent. The only group who still seem to use the figure of 88% are the Bicycle Helmet Initiative Trust, who (no doubt purely by coincidence) have apparently been working with Halfords to try to bolster their campaign for helmet compulsion. BHIT are well aware that the figure is wrong, and why, and have been reported to the Advertising Standards Authority for using it, but they apparently do not mind misleading a large retailer.
