Tuesday, 14 August 2012

How to call bullsh*t on statistics part whatever

One of my first ever blog posts was noting a long-running error in the Economist about US car ownership. Here a similar version appears in The Atlantic. The claim is that US - so associated with car ownership - has fewer cars per 1,000 people than Europeans.

The claim is not wrong, as the figures provided show. But it is misleading, for the simple reason that in the US SUVs, or pickup trucks, which are predominately used for private transport, ie as a car, are counted as "light trucks". In Europe they are not. In both markets these vehicles have taken market share since the 1980s.

The first chart (apologies for the formatting and layout here) shows US car ownership per 1,000 people over time. It shows that car ownership peaked in the mid-1980s at about 550 cars per 1,000 citizens, and has steadily declined since to about 420.

The second chart explains why this has happened. It shows the number of "trucks" per 1,000 people, which has risen steadily to over 350 by 2010.

 Finally the third chart puts them together. The cars have declined, the trucks have risen, and the total has - as you might expect - steadily increased.

It should be said that some of these "trucks" are what we in Europe would call trucks - vehicles for carrying cargo. But the amount is relatively small - each year about 500,000 heavy trucks are sold, compared to 6 million or more light trucks.

The reason I spotted this one was simply because I have researched this area quite a bit over the years. But if there is a lesson, it's probably that statistics that don't seem to fit in with what you know about the world around you should be checked thorougly.