vitualis wrote:The point is that this weakens their data analysis. If pre-existing helmet wearing was already common without a law, the institution of a mandatory law is likely to have a lesser effect on ridership. Similarly, if after the implementation of such a law, the rates of compliance to the law is low, it is also going to have a lesser effect on ridership.
Optimally, you want the study to be done in a study population that has low helmet wearing prior to the law and then good compliance with helmet wearing after the law comes into place.
why? surely the objective is just to assess the impact of helmet laws on ridership rates. if the impact is low and explained partly by low compliance with the law, then the impact is still low. that's a perfectly legitimate finding, to me, unless the effect of the law is different in the region you wish to apply the results to. obviously if the law isn't improving wearing rates, that's a problem in its own right.
vitualis wrote: Actually, you're the one who has it the wrong way around. The onus is on the authors to be able to justify their conclusions on the basis of their study. Their conclusion is that the introduction of helmet laws in Canada "was not associated with changes in ridership". My reading of their study is that there are problems with their methodology such that this conclusion may not be validly justified (this is not to say that this conclusion is actually incorrect, just that they haven't adequately demonstrated it).
you're entitled to that view and you could even be right, but frankly any academic study will draw criticism, particularly from those who don't favour the conclusions. i'd be interested to see credible studies with contrary findings. their apparent absence speaks more to me than your concerns about the robustness of this study.
vitualis wrote: I have some sympathy for the researchers because this type of research is extraordinarily hard to do. For example, the response rate in their surveys (which mind you the authors haven't commented on their reliability because it most likely wasn't tested --> this is also a potential source of substantial bias due to problems with memory recall) is actually pretty good.
sourcing and processing the data is a problem with many statistical studies - i understand your concerns (broadly). but again, the hypothesis is that helmet laws have reduced ridership rates. that's just not proven, even if it's become a mantra of helmet law opponents.
vitualis wrote:Addit: you mentioned that they could "hire helmets" with the bikes. Actually, this doesn't work for a number of reasons: (i) the scheme use a large number of automated hire points around the city and (ii) hygiene problems with reusable helmets and the practical issues of cleaning helmets (I think this is the reason the Melbourne scheme doesn't hire helmets). Disposable folding helmets might be a solution but I assume this will substantially increase the cost of such a scheme.
it's a problem. disposables, which were suggested in a weekend newspaper article on the topic, are unlikely to meet the mandatory Australian Standard. like much of the article, they didn't think that out very carefully..
surprise, surprise.
