James Ball

The R-number – and the danger of false certainty

The R-ometer

Not much about Boris Johnson’s Sunday night television address was clear. The one definite new measure – one which will shape coverage for weeks to come – is the UK’s new ‘COVID Alert Level’, a five-stage measure that the prime minister said would be determined primarily by ‘the R’ – the rate at which the virus grows.

There’s just one problem with that. It’s a figure few people think we have any real ability to track day-to-day. In making its decisions, the government has very little scope for error: it knows lockdown comes at a huge economic toll. But it also knows that spotting a rise in cases just a few days too late could end in calamity, if it lifts restrictions too early. The R number was presented as a solution to that, something whose precise measurement will determine government policy. The Prime Minister said he’ll be driven by data – but is he being honest about how reliable the data is?

At the moment, we hear a lot of very precise numbers at government daily. We hear an exact figure in the tens of thousands about how many new tests have been conducted. We hear a number of new cases in the thousands. And we hear a number of deaths, still in the hundreds. These appear definitive. They’re being obtained by dint of great effort and announced as official government figures by senior ministers every day. But one person left largely unimpressed by them is Professor Sir David Spiegelhalter – and you might expect him to be onside, given he’s the professor of public understanding of risk at the University of Cambridge.

There is a real risk that with the false level of certainty they’re projecting, Boris Johnson’s government is talking out of its Rs.

Spiegelhalter’s concern, shared over the weekend with the BBC, is that the level of precision being given is a false one.

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