Tim Harford’s books are on the reading list for journalism students at Jomec and we are big fans of More or Less. And this month he supplied us all with a great case of numbers going wrong, in a piece for the Financial Times.
You can listen to him explaining it on Radio 4’s The World at One (segment at 17′ 52″).
The thinking — about how dangerous UK life is during the Covid pandemic — goes like this:
- Every day in the UK about 40 people out of a million get the virus (ONS).
- How dangerous is it if you’re one of the forty? If you’re aged 60, you have roughly a 1% chance of dying if you catch it.
- 1% of ’40 in a million’ gets you to almost a 1 in 2 million chance of dying. So, if you are 60 and live in the UK at the moment (and are exposed to the typical risk in the UK) there’s a 1 in 2 million chance Covid will kill you.
- Or make that a one in a million chance if you include ‘serious injury’ since another 1% of the ’40 in a million’ who catch it are left with health problems.
Everything, Tim Harford says, is fine up to here. But then he looked for other things that had a one in a million chance of death / serious injury. One of them, he explained to The World at One, was “taking a bath”.
“So when I discovered this I thought ‘oh, I wonder what else is about that risky?’ […] So when I wrote this all up for the Financial Times I just — as an afterthought, having worked so carefully to get all my Covid maths right — I just said ‘it’s a bit like riding a horse, riding a motorbike, going skiing, or taking a bath'”.
This is the error. The risk of dying in the bath is one in three million every year — not every time you take a bath. As Tim Harford remarks “Covid is no more risky than you thought. And taking a bath is much safer than you thought”.
Nonetheless, “That is the most shared thing I’ve ever said because it’s the most interesting thing I’ve ever said […] because it happens to be wrong”.
It is, as he observed, an instructive case of how mistakes happen and what newspeople pick up on.
His full account of it is on twitter.