Friday, 29 August 2008

Telling stories and lies with statistics

An article in the 30/8/08 New Scientist "Keep your head" discusses how our emotions override rational decision-making.

It is fine as far it goes, and the example of the vast increase in deaths as a consequence of people driving long distances instead of flying, due to fear caused by the terrorist attacks, is fine. But, the chart that it uses to support the argument - and the general story-line of "when it comes to risks, feel the numbers" - raises other problems.

Here is a reproduction of the chart:

OK, so flying is much safer than driving. But, look, driving is much safer than walking or cycling? Is that right? Can I really say 'driving is much safer than walking or cycling' based on this graph? I don't think so, and let me give three problems with drawing simplistic conclusions like that:

1) They are not simple alternatives. You can't choose between flying or walking to the local shop. If you need to do some food shopping you might choose between walking ten minutes to the corner shop, cycling ten minutes to the 'mini-market' or driving ten minutes to the supermarket. The comparison then is between deaths per hour, not deaths per kilometer. You could make a similar choice for you holiday destination: drive, train or bus to Blackpool, or fly to Spain. Again, the meaningful comparison is per hour, not per km.

2) These only include deaths to the passenger during the travel. What about killing other people? Cars kill other people - including cyclists and pedestrians, bicycles rarely do and pedestrians never do (not as a result of walking into people anyway - I presume!). And of course cycling and walking keeps you fit. It is said that the health benefits of cycling far outweigh the increased risk from accidents, compared to driving. I don't have the figures to confirm that, but it is a consideration.

3) What about wider systems issues - damage to the environment (global warming), impact on the economy. These are really difficult to evaluate, but it doesn't mean that don't exist.

To look at (1), I did a rough and ready conversion of the numbers from the article to 'deaths per million hours' based on a guess of the average speed for each mode of transport. (Motorcycle, car and bus 70 km/hour, walking 4 km/hour, bike 15 km/hour, van 60 km/hour, water 50 km/hour, rail 100 km/hour and air 800 km/hour. I also needed a figure for deaths per billion km for air travel, because the New Scientist article just said ‘less than 0.1'. A search on the web turned up a figure of 1 death per 15 billion passenger km, so I used that.) This gave the chart below (note that the column for motorcycle is shortened).

I admit I was disappointed to find that cycling still came out worse than driving, but the difference is now much smaller, and surely more than compensated by the health benefits of cycling. The thing that surprised me in this, was that flying came out worse than water, bus or rail. So a train trip to Blackpool looks like your best bet for the hols! (And why would anyone ever go anywhere on a motorbike!)


Robin Faichney said...

Another factor to consider: most road accidents happen in built-up areas, during short journeys, near home, so modes of transport that are used more for shorter journeys are disproportionately represented in the statistics. I first came across this in connection with motor cycling but it obviously applies also to bicycling and walking. I believe journey length/accident figures might be available but I'll leave it to you to hunt them down!

David Chapman said...

Thanks for the comment. Yes, similarly, in a comment to the New Scientist article on their website, someone else made the point that by far the most dangerous time in a flight is take-off and landing. I think the message is just that numbers/statistics are very difficult to interpret but are meaningless without interpretation (without a supporting storyline). This is not to deny the message of the article, that you should look at the numbers and be careful of emotion, but it is a long way from the whole story.