I recently read Emily Oster's Cribsheet, her data-driven examination of the endless recommendations about parenting young children. Her primary tactic is determining the validity of study methodologies in order to understand the strength of various statistical conclusions. Her consistent refrain is to be careful with any data that does not separate the type of person from the parenting decision because no researcher can ever conclude from such studies whether the type of person mattered more than the decision in terms of influencing the observed outcome.
The book got me thinking about the ways I’m exposed to phony analytics. The bike helmet is a great example. Is the type of person who uses a helmet different from the type of person who does not use a helmet? I'd say yes, and would like to know the extent to which the safety gains are due to the nature of the person rather than the helmet. The question, unfortunately, has many layers. The helmet itself isn’t a going concern until there is some kind of head impact, and in those moments it's clear that you'd rather have the helmet. But do helmeted riders crash more often? Some researchers have studied whether wearing a helmet makes a person more likely to get hit by a car, generally exploring one of two hypotheses: (a) the helmet invites riskier driving or (b) the helmet encourages riskier cycling. It's tough to answer these questions and I'm grateful for those like Emily Oster who do their best to educate us with the truth.
One angle I haven't heard much about is how helmet ownership changes riding decisions. I'll offer my story as anecdotal evidence – after getting a helmet, I've felt less safe without it, and this had led to fewer rides. (Those in lockdown may be able to relate to a different version - having to wear a face covering, I feel less safe, and I go out less often.) I ride less often because when I forget to bring my helmet with me, I don't ride a bike, whereas in the past since I never 'forgot' the helmet I didn't own, in these situations I would have pedaled on as was my original intention. If we assume that bike accidents happen with equal probability regardless of helmet use, one sure way to lower the number of accidents is to ride less, and people who feel less safe will ride less.
Again, I simply pose the question of whether helmet use increases safety by emphasizing danger, which leads to fewer bike rides – I’ll leave any official verdict to the experts. It's not going to change my decision to ride with a helmet. But from my point of view, whatever the perceived benefit, it surely is being exaggerated by ignoring the marginal decisions made by people like me. The failure to account for such an effect is the exact sort of thing that leads people to (correctly) question the accuracy of the metric, and although the difference may be insignificant, whenever doubt is cast on the truth, the skeptics have an opportunity to misrepresent the error, discredit the overall idea, and make it more difficult for others to make good decisions for their health and safety.
