The media, politicians and interest groups love to throw numbers at us to support their cause - and they can be very useful - but we always ought to ask: 'Where did those numbers come from?' and 'What does that actually mean?'
Here's a good example. In 2015, a website attacked the UK Labour Party's policy on arts funding pointing out 'Just so you know, arts funding brings in £4 for every £1 spent.' It's quite likely this statistic will get a life of its own and be used many times in the future. But what does this actually mean, and where did the numbers come from?
As far as a source, the figure seems to be genuine. After following a chain of different people using it, it originated in a 2013 report by Arts Development UK. So I was, with a little work, able to check where it came from. Yet the chances are, when a figure like this is used, it will often not be possible to easily find out its exact source.
And what does it actually mean? I asked quite a few people who all, like me, assumed that the claim 'arts funding brings in £4 for every £1 spent' meant that if you fund the arts, for each £1 you spend, £4 will come in either directly (through entrance fees, brochures etc.) or indirectly (for instance by more tourists coming and spending money in your town or city). But look at that original report and you'll find something entirely different was intended.
The report says 'For every £1 spent by local authorities on arts service, leverage from grant aid and partnership working brings in £4.04 of additional funding.' So, when a local authority spends £1 it gets that additional money from grant aid and partnership - which is largely with other authorities and with bodies like Arts Council England. In other words, £1 of local government spend brings the local authority another £4 of public money. Excellent for the local authority, but hardly supporting the argument in which it was used. Whether intentional or (probably as here) accidental, this is a kind of deception.
We usually don't have the time to trace back a statistic to its source like this - but don't be taken in by numbers picked out of the air. Anyone who uses numbers like this should expect to have them challenged - and needs to be able to show exactly what they mean and where they come from.