The biggest helmet lie

From ChapmanCentral

The biggest lie told by helmet promoters is that "85% of head injuries" or "88% of brain injuries" can be prevented by cycle helmets.

This is a real whopper on many levels.

• It is a misrepresentation. The odds ratio is presented as meaning that helmets prevent this proportion of injuries, but this is sophistry. because odds ratios tend to overstate relative positions. 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. Using the TR&T approach men are 36 times more likely to have drunk wine, but actually men are only 4.5 times more likely to have drunk wine than women.
• It is made up anyway. The figure calculated from the measurements and populations was lower, but the authors made an estimate of the effect of under-reporting and upped the figure.
• It is the highest figure ever suggested, and has never been repeated. It comes from a single study, Thompson, Rivara and Thompson (1989), which reached its conclusion by assuming that the only difference between the risk profile of solo urban cyclists and middle-class people riding in family groups on bike trails, was helmet use.
• Every other study, including the authors' own reworking of their own figures, is substantially lower
• It equates correlation with causation. Consider: ugly and socially inept people are less likely to be married, so does being married reduce your chances of being ugly and socially inept? I'd say it was the other way around. So: is the find that middle class riders on bike trails are less likely to suffer head injuries than teenage street kids, down to the fact that the street kids don't wear helmets? Or is it that the population that is less disposed to take risks is also, coincidentally, more disposed to wear helmets? So: whatever you can say from this study it absolutely does not establish a causal relationship, as is routinely (mis)represented.
• Real-world evidence contradicts it. Prospective studies that predict benefit should be tested against real-world evidence. This has been done up to a point (see the cycle helmet debate); a few countries have introduced helmet laws which have seen substantial and rapid rises in helmet use. If the preventive effect was anywhere near this level, it would be immediately visible in the statistics. It's not. Quite the opposite:

If you can see any evidence of meaningful effect here then perhaps you could drop me a line. Not only is the trend for cyclists largely indistinguishable from that for pedestrians, there is no sign at all of any reduction to match the huge rise at the time of the passage of the helmet law. So to continue to claim, as the helmet advocates do, that prospective studies are predictive of the injury savings that would be yielded by increased helmet use, is to completely ignore a substantial and important conflict of evidence.

And all this is before you even start to consider the vexed question of what constitutes a head injury.

The fact is, the fact that the 85% / 88% figures are bogus is well known; an honest helmet promoter will not use these figures. But the most ardent helmet promoters continue to use them, and to assert that laws, if passed, will yield these benefits, despite the fact that no law yet passed has yielded any provable benefit.

In the end, then, anybody who says that helmets prevent 85% of injuries or 88% of brain injuries is either woefully ill-informed, or lying. There are no other options.