Skip to main content

Correlation street

All too often we see a story in the newspaper where numbers are painfully parroted without giving any consideration to what they actually mean - and all too often that means we need recite our favourite mantra, 'correlation is not causality'.

Today's paper carried a wonderful example of this, citing 'research' by Lloyds Bank showing that living in the vicinity of a supermarket will have a varying impact on the price of your house 'depending on the status of the shop.'

As we all know, it shows nothing of the kind. There may be a correlation between being near the shop and house prices - but it's highly unlikely it's causal. The reason we can be reasonably sure of this is that occupant of the number 4 position, Iceland. Anyone who knows their 'status of the shop' rankings knows that Iceland is the pits - certainly below Asda. I don't doubt the pulling power of Waitrose, but the fact is I'd suggest there are other causal factors at play here.

Why would Iceland score higher than the Tesco to Asda set? I'd suggest because these lower rated shops are more likely to be located in the suburbs, where Iceland is more likely to be in a town/city location with higher property prices. Of course I don't know that this is true - it would take a lot more work that I (or Lloyds, I suspect) am prepared to put into it. But I think a third factor, such as location, is far more likely to have produced the Iceland oddity than status. (Location could also impact quite strongly on, for example, M&S.)

News media - I'm not asking you to go into detailed statistical analyses (though that would be nice) - but please think about whether a story is about correlation or causality before you tell us that X causes Y. You wouldn't publish a story with a total guess of who won the FA Cup Final - don't give us guesswork causes hidden behind numbers.

Comments

Popular posts from this blog

Is 5x3 the same as 3x5?

The Internet has gone mildly bonkers over a child in America who was marked down in a test because when asked to work out 5x3 by repeated addition he/she used 5+5+5 instead of 3+3+3+3+3. Those who support the teacher say that 5x3 means 'five lots of 3' where the complainants say that 'times' is commutative (reversible) so the distinction is meaningless as 5x3 and 3x5 are indistinguishable. It's certainly true that not all mathematical operations are commutative. I think we are all comfortable that 5-3 is not the same as 3-5.  However. This not true of multiplication (of numbers). And so if there is to be any distinction, it has to be in the use of English to interpret the 'x' sign. Unfortunately, even here there is no logical way of coming up with a definitive answer. I suspect most primary school teachers would expands 'times' as 'lots of' as mentioned above. So we get 5 x 3 as '5 lots of 3'. Unfortunately that only wor

Why I hate opera

If I'm honest, the title of this post is an exaggeration to make a point. I don't really hate opera. There are a couple of operas - notably Monteverdi's Incoranazione di Poppea and Purcell's Dido & Aeneas - that I quite like. But what I do find truly sickening is the reverence with which opera is treated, as if it were some particularly great art form. Nowhere was this more obvious than in ITV's recent gut-wrenchingly awful series Pop Star to Opera Star , where the likes of Alan Tichmarsh treated the real opera singers as if they were fragile pieces on Antiques Roadshow, and the music as if it were a gift of the gods. In my opinion - and I know not everyone agrees - opera is: Mediocre music Melodramatic plots Amateurishly hammy acting A forced and unpleasant singing style Ridiculously over-supported by public funds I won't even bother to go into any detail on the plots and the acting - this is just self-evident. But the other aspects need some ex

Which idiot came up with percentage-based gradient signs

Rant warning: the contents of this post could sound like something produced by UKIP. I wish to make it clear that I do not in any way support or endorse that political party. In fact it gives me the creeps. Once upon a time, the signs for a steep hill on British roads displayed the gradient in a simple, easy-to-understand form. If the hill went up, say, one yard for every three yards forward it said '1 in 3'. Then some bureaucrat came along and decided that it would be a good idea to state the slope as a percentage. So now the sign for (say) a 1 in 10 slope says 10% (I think). That 'I think' is because the percentage-based slope is so unnatural. There are two ways we conventionally measure slopes. Either on X/Y coordiates (as in 1 in 4) or using degrees - say at a 15° angle. We don't measure them in percentages. It's easy to visualize a 1 in 3 slope, or a 30 degree angle. Much less obvious what a 33.333 recurring percent slope is. And what's a 100% slope