This post is part of a series: it starts with the intro to the helmets issue, then the summary of the best evidence on helmets, then a quick diversion into how dangerous cycling is. And it won’t end here…
A good review of a medical intervention starts by explaining the population being studied. The Cochrane review of helmets for preventing head injuries in bicyclists explains that its population is the set of bicyclists who sustained an injury that was sufficiently major to make them go to the ER for treatment (and not sufficient to kill them before they could seek treatment).
The review does not explain what they mean by a bicyclist. (And since the original papers under review are closed-access, behind an extortionate paywall, we can’t know whether those do.) Presumably they mean people riding a bicycle at the time that they sustained their injury.
Is that people riding their bicycle leisurely along a rail trail or towpath?
Is that people touring, head down into the wind in the deserted mountains?
Is that people racing in a peloton down the dual carriageway?
Is that kids in the BMX park?
Is that mountainbikers on the downhill courses?
Is it businessmen on their Bromptons riding through the stop-start city traffic? Old ladies bouncing down cobbled streets on their step-through upright bikes? Village vicars doing their rounds?
Mountainbikers, city commuters, and rail trail riders are very different people exposed to completely differently environments and risks — and who have very different helmet-wearing and hospitalisation rates. Lumping them all together is like lumping mountain hikers, sprinters, traceurs, marathon runners, city pedestrians and country footpath strollers together under the heading “walkers”. But lump them together is exactly what the studies in the Cochrane review do, comparing the rate of head injury (as a proportion of all injuries) in helmet wearers and non-helmet wearers, and applying the results to make the recommendation that everybody should be made to wear a helmet while riding a bicycle, whatever their style and situation. You may as well recommend Formula 1 safety gear for the drive to the supermarket.
Perhaps helmets help prevent head injuries in all people who use bicycles. Perhaps it helps mountainbikers more than tourists. Perhaps it’s the other way around. We don’t know. We could know. The researchers could have made sure to collect the data (perhaps the data is even already there, in the medical records) and then done sub-group analyses on their data to give individual results for separate groups of bicyclists. But they didn’t. Why not? Did it just not occur to them that “bicycling” might not be a single pursuit? Or did they just assume that it didn’t matter, or that nobody would notice? Either way, it amounts to a pretty serious limitation when you’re asking “should we legislate to ban all kinds of bicycle use except where the bicycle user is wearing a helmet?”
At War With The Motorist 
The voice of reason. Thank you so much for this. I thought I had heard all of the arguments and given this issue serious and critical thought. I hadn’t of course. Your last sentence is a very important point. Helmet laws, as we have over here in Australia, effectively amount to a ban on all bicycle use unless the rider is wearing a helmet. That, of course, is exactly what has happened.
To not take into account the way in which bicycles were being used when injuries were sustained seems a bit remiss, particularly when there may well be a bias towards the documented injuries having been sustained in the more ‘extreme’ (and probably rarer) forms of bicycle use. As you say, it would be like including data from motor racing when considering advice for the protective clothing motorists should wear when going to the shops – absurd, yet that’s the sampling method in operation in this review.
FWIW, here’s my superficial take on the ‘debate’ – http://bit.ly/gBx7dr.
Ah, so that is your take on it…. Many other problems with the Cochrane review though, some of which you mentioned in the previous post. There is another Cochrane review that looks specifically at the effect of helmet compulsion laws on a population level. That should be far more relevant, as then issues of risk compensation and more come into play. First author Macpherson.
There are several issues with that study too, and your issues with using a Cochrane report as the single input and forgetting about absolute risks etc, etc of course still apply. Here is the Bicycle Helmet Research Foundation take on that study : http://www.cyclehelmets.org/1181.html