by Tom Chivers
Recently, I was lucky enough to win the ‘Explaining the facts’ category in the Royal Statistical Society’s Statistical Excellence in Journalism awards. In my brief acceptance speech, I used a quote I half-remembered. I don’t know who said it, but it goes something like this: if you want to fool people, the easiest way to do it is to never teach them what a denominator is. As a journalist, one who’s interested in science and facts and numbers, I think it is something to remember at all times.
It might sound a bit abstruse. The denominator is the number below the line in a fraction; the “4” in “¾” or the “8” in “⅝”. Not knowing what that is might not seem obviously devastating to your ability to navigate the modern world.
But I think it is. What it means is: numbers need context. Say you read a story saying that the FTSE “drops by 100 points”. Is that a lot? A little? You don’t know. If the FTSE was previously at 200 points, it probably is; if the FTSE was at a million points, probably not. The denominator – what the FTSE is actually at – is the difference between the story being big and the story being unimportant.
Or, say, you read that on average 14 cyclists are killed on the roads in London a year. In the absence of the information that about 700,000 people cycle every day in London, you can’t make a good assessment of the risks. It’s just a number, divorced from everything.
The whole of the media is guilty of making numbers incomprehensible by stripping them of context, and I don’t imagine it’s usually deliberate. I regularly see headlines like “X trans women are murdered per year” or “Y people die in police custody”. The question that needs to be asked of these headlines – and all too often isn’t – is are those big numbers? We can’t tell without context – is that more or less than you’d expect given how many trans women there are, or how many people are actually in police custody?
This “denominator blindness” isn’t the only problem; any bald number can be misleading. For instance, relative risk. If you tell me that, say, eating bacon every day raises my risk of colorectal cancer by 20% if you don’t tell me what my risk of colorectal cancer is in the first place. What I need to know is the absolute risk: what would my chance be if I don’t eat bacon, and what would it be if I do? (The answer is about 5% and about 6%, respectively.) I can’t make good decisions about what to do with my life without context for the numbers, without asking is that a big number.
That’s why the job of the journalist has to be more than just reporting the bald facts, the unadorned numbers. We are supposed to be helping people navigate the world, I think. If we find a number, we need to help put that number into context. Our job needs to be to learn what a denominator is, and to teach it to people. I was thrilled to win the RSS award, because it was a vindication of my work – but more importantly, they exist to remind, and encourage, journalists to take the numbers they have to deal with in almost every story they write, and to contextualise and explain them.
Tom Chivers is an award-winning science writer. His first book, The Rationalists: AI and the geeks who want to save the world, will be published by Weidenfeld & Nicolson in 2019. (Image credit: Buzzfeed)
Tom won the ‘Explaining the Facts’ prize at the Royal Statistical Society’s Statistical Excellence in Journalism awards earlier this month, which were sponsored by the ESRC.
You can follow @TomChivers on Twitter.