It’s good to see some statisticians have taken a look at Gillham and Rissel’s surprising (and depressing!) claim that per capita cycling levels have fallen significantly in Australia over the last 27 years.
As I discussed last month (Are Australians cycling less?), Chris Gillham and Chris Rissel reported recently that the number of cyclists fell on a per capita basis “by 37.5% between 1986 and 2011.” They estimated this outcome by comparing the results of a mail-out survey undertaken in 1986 against a telephone survey done in 2011.
Professor Rissel says the drop was most likely due to the mandatory helmet legislation introduced in Australia in the early 1990s. “Well over a half a million more Australians could be riding bicycles”, he says, “if we didn’t have mandatory helmet laws.”
A group of researchers led by Jake Olivier from the School of Mathematics and Statistics at UNSW challenges the significance of that finding in an article published yesterday on The Drum.
Using the same data, they confirm the conclusion I drew last month (Are Australians cycling less?) that Gillham and Rissel significantly over-estimated the extent of the decline. The correct figure isn’t 37.5%, it’s 22% (I estimated 24% but I’m not going to argue).
But Olivier et al reckon there’s an obvious explanation for the per capita fall (no one disputes cycling rates increased in absolute terms over the period). They say it can largely be explained by the ageing of the Australian population over the 27 year period between the two surveys.
That’s because people cycle less as they get older. They say:
After adjusting for the overall ageing of the population between the 1985/86 survey and the 2011 survey, the per capita cycling trip rate has actually increased by eight per cent.
As I read it, Olivier and his colleagues aren’t disputing that the surveys cited by Gillham and Rissel show cycling fell in total in per capita terms (i.e. by 22%). Rather, they’re saying it probably doesn’t reflect a loss of interest in cycling or a failure of policy, but is most likely due to exogenous demographic changes:
The apparent drop in the crude cycling trip rates reported by Gillham and Rissel is more than adequately accounted for by demographic shift in our ageing population. There is no need to speculate about lack of investment in cycling infrastructure as a cause for a decline in cycling participation, and absolutely no need to invoke the bogeyman of mandatory bicycle helmet laws.
So in a sense both parties are right. But it’s a like a theological debate because it takes it on faith that valid conclusions can be drawn from the two surveys. As I said before (here), I don’t think the data is good enough to draw definitive conclusions. Olivier et al acknowledge that point:
Of course, we should not put too much store in any of these figures – the two surveys on which these calculations were based differ in several important ways, and like all such surveys, each has its own margin of error, which we haven’t been able to estimate from the information available.
Not surprisingly, discussion around the Gillham & Rissel and Olivier et al articles is already mired in an often intense and vitriolic debate for and (mostly) against the mandatory helmet law. Whatever the merits of each side of that argument, the source data relied on here is too flaky to support (or undermine) either position.