Thursday, 9 September 2010

Numbers in the news

When people ask me about statistics they hear on the news, I suggest two questions they ought to bear in mind, and as a result of which they should employ appropriate scepticism. Those questions are:
  1. What does that mean?
  2. How do they know that?
Let me demonstrate. You hear there has been a 100% increase in a particularly nasty crime over the previous year. That's horrendous. The world is unsafe. We'd better legislate. But hang on. What does that mean? 100% of what? It turns out the previous year there was one instance. This year there were two instances. Stand down the national guard.

Another example to deploy question 2. I heard recently on the news that exports were up by so many percent over the previous year. How do they know that? I have several activities that count as exports. For instance, one of my main publishers is St Martin's Press in New York. As a result of selling them my books, money flows into the UK. This is an export, even if a physical object doesn't get popped in the post. But will that show up in their statistics? I can't see how. There must be many thousands of small businesses, exporting physical goods for internet sales, for example, where again I can't see how those sales will get into those export figures. The fact is, though they don't admit it, that this must be a guess - and not a very good one.

1 comment:

  1. A few months ago, the BBC News site promised to explain these sort of percentage figures they use.

    For example:
    Eating bacon increases the risk of some cancer by 20%

    Sounds bad?
    Not until you work out the figures properly. (I don't have the exact figures to hand, but it's something like...)

    If 5 people per 100 get this cancer, then if everyone ate bacon every day, then the figure would be 6 people per 100.

    So, the 20% is somewhat misleading.

    However, since the BBC News site made that promise, I've yet to see a single additional explanation...