For the record I am all for debate. My perspective on the points you raised
rfryer wrote:Common sense, reason Thinking about why a helmet can have positive, or negative effects on safety.
I dislike the term common sense. Rational hypothesis is a vital starting point for scientific inquiry. It is important that we understand how helmets are meant to work by compressing in the event of impact. Likewise it is important to understand the reasoning behind counter arguments that helmets might make injury more likely through increased risk of impact, rotational brain injury and psychological effects such as risk compensation. Bear in mind that reasoning is not the same as evidence. the history of science is filled with well reasoned hypotheses refuted by evidence.
Personal experience, & anecdotal evidence from others Especially when they confirm, or refute, common sense arguments. For example, I've see a lot of "helmet saved my life" claims, but not very many "helmet nearly killed me" claims.
It's important to understand the limits of personal experience; "I banged my helmeted head and did not sustain an injury" is personal experience, "my helmet saved my life" is conjecture. As I think you're implying, people are also inclined to rationalise their own decisions so there will be a tendency for people to interpret their experiences in a way which supports their choice. As it inevitably relies on one or a small number of individual events which will necessarily be unique and unrepeatable, I consider this a relatively poor source of information.
Knowledge of local cycling conditions, and personal cycling style This is critical; the effect on your safety of wearing a helmet depends very much on where you cycle, what you cycle, and the balance of risks in the way you ride. The answer will vary for different people, and even for an individual at different times.
Expectations Would you consider wearing a helmet purely to increase your chance of surviving a major trauma? Or is it worth wearing simply to reduce the chance of cuts and grazes?
I'll lump these two in together. Yes, it is vital that people have an understanding of what they can reasonably expect from their helmets and how their type of cycling and the conditions under which they ride might affect their risk profile. There is, however, a well documented difference between the perceived and actual risk of cycling, so it is not unwise to keep one's impressions in check with reference to actual levels of cycling injuries.
Statistics This is where I disagree with a lot of forummers; I'm not prepared to put much weight on statistical evidence, for two fundamental reasons. Firstly, I don't believe the statistics out there are based on complete data - for example, I have had a number of falls, with and without a helmet, and none of them are tracked in any statistics.
And this is where I'll have to disagree with you. All falls may not be recorded, but pretty much all serious head injury and definitely all fatal head injuries will be. Given that these are the incidents for which the boldest claims about helmets are made, they should provide a good source of information. It seems daft to value anecdotal evidence - which ultimately amounts to a single data point - highly yet dismiss statistics because they are necessarily incomplete.
Secondly, even if they were entirely trustworthy, and said with authority "on average, safety (both in terms of death, and injuries of all severities) is reduced (or increased) by 5% by wearing a helmet", how would that help me in deciding whether to wear one tonight, on my very specific, completely non-average ride?
May I cheekily retort, how much help would the 'anecdotal evidence from others' you value be in you deciding whether to wear one for your very specific ride?
'On average' figures are most useful when considering helmet policy for all cyclists rather than individuals. Still, it might help somebody to know how much difference helmets make for the average cyclist, though, obviously, the more you deviate from the average the less useful such a figure would be. I still see average effects as useful information regardless of the fact that we cannot model each individual cyclist's risk profile.