E-bike commuting, happiness, and survivorship bias

If I told you that e-bike commuters are happier than drivers are, would you conclude that travelling to work by e-bike made people happier? Perhaps you would, but if you did, you would be confusing correlation with causation. Perhaps happier people are more likely to commute using e-bikes? Or perhaps only higher-income people can afford an e-bike, and higher-income people are happier? It is difficult to say. However, jumping straight from the observed correlation into studying why e-bike commuters are happier shouldn't be your next step, especially not if you are going to base your study on talking to only 24 e-bike commuters.

However, that's exactly what this study published in the Journal of Transport & Health (sorry, I don't see an ungated version online), by Kirsty Wild and Alistair Woodward (both University of Auckland), did. It was covered in the New Zealand Herald earlier this year, but I held off on writing about it until I had a chance to read the research myself.

The problem isn't so much the research itself - interviewing e-bike commuters about what they like about e-bike commuting is fine. However, extrapolating that to answer the question about what should be put in place to encourage e-bike commuting, as this study does, is fraught. The reason is survivorship bias.

Almost by definition, if you interview current e-bike commuters, then you're interviewing people who tried e-bike commuting, and liked it. However, there are a bunch of people who tried e-bike commuting and hated it - they don't commute by e-bike any more, and they didn't get interviewed. In other words, the current e-bike commuters are the survivors from a larger group of people that have tried e-bike commuting at some time.

The problem in this case is that those two groups (survivors and non-survivors) are different. At the very least, the survivors like e-bike commuting, and the non-survivors don't (or, at least, they don't like it enough to continue commuting by e-bike). Interviewing the survivors tells you nothing about what the non-survivors liked or didn't like about e-bike commuting. It could be that the things that the survivors like about commuting by e-bike are exactly the things that the non-survivors hated about it. And you can't tell from this research, because former e-bike commuters (the non-survivors) were not interviewed.

So, if you decide to base decisions about cycling infrastructure on what current e-bike commuters like about it, there is no guarantee that e-bike commuting would increase as a result. The people who want to commute by e-bike with the current infrastructure are already commuting by e-bike. What you really want to know is, what do the people who don't currently commute by e-bike want?

I'd find research like this a lot more plausible if they had interviewed people who gave up on commuting by e-bike, or if they interviewed both groups. Or, even better, if they ran an experiment where they distributed e-bikes randomly to some people and asked them to use them for commuting, and then interviewed that experimental group about what they liked and did not like.

Otherwise, you simply get more of what the survivors already like, and don't necessarily create the right environment to increase e-bike commuting at all.