Friday, 30 September 2011

Numbers Are Fun

I live in The Forest Of Dean in Gloucestershire, and you couldn't find a finer place to live. 

You could however find a finer place to drive. The roads are notoriously bad for a number of reasons; lots of blind brows to sharply rising hills, hair pin bends with cliffs, no lighting, hidden dips, boar, deer, and sheep lots and lots of sheep. So when the Gloucestershire Road Safety Partnership erected signs on the main A48 through the area warning how dangerous it is then you'd think that I'd be happy. 

Weeeeelllll.... No.

It's not the signs, or really even the sentiment, it's the woolly reporting of numbers that annoys. The signs tell us about the number of Casualties on the road in the last 3 years. It's reasonably high at 39, it averages out at 13 a year. However when you do a little bit of research it turns out that these signs have replaced temporary ones that warned of 39 collisions on the road in the last 3 years.

Collisions or Casualties? 

Casualties in this sense is hugely emotive. If you take it at its most extreme definition then it could mean that there have been 39 deaths... I've had a look back over the last 3 years for reports of deaths on the A48. The Gloucester Citizen has been invaluable for this research (and they really know how to string out a good road death, coverage can last for months!) and I've managed to come up with.... 9 deaths since June 2008. 

9 is still a bad number, but then you look at the inquests; 1 black ice, 2 Drugs, 1 drink driving and a trip to prison, oh and 1 motor bike. So 5 out of the 9 have reasons beyond being careful and 1 of the remaining 4 deaths had excessive speed mentioned as a cause of the accident. 

So what am I saying here?

To use '39 casualties' isn't wrong it's just a little miss-leading (depending on the interpretation of casualty). If someone had a low speed shunt and was taken to hospital they then ended up on the casualty list. So it could be that there have been 9 deaths and 30 very minor accidents that meant a trip to hospital just to be on the safe side. 

It's very difficult to stop numbers having some sort of emotional baggage. It's not the numbers fault, they are entirely blameless, we're the ones that make them dance for us. Numbers when used to support a particular point of view (in this case that the A48 is dangerous even though it's categorised as Low/Medium by EuroRAP http://www.eurorap.org/) need to have context. I'm not even going to start talking about regression toward the mean that's for another day http://en.wikipedia.org/wiki/Regression_toward_the_mean

So next time you hear someone boast that their sales have risen by 100% year on year, just remember that if you add context it could mean that they only made 1 sale last year and 2 this.

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