The rule of four: how to make sense of Covid case numbers

Are Covid cases doubling or not? And if so, in what time frame? If you listened to Boris Johnson and chief scientific adviser Patrick Vallance, you’d be forgiven for being confused. The Prime Minister said this week:

‘The chief medical officer and chief scientific adviser warned that the doubling rate for new cases could be between seven and 20 days.’

In fact, Vallance had said this:

‘At the moment we think that the epidemic is doubling roughly every seven days.” And added, “It could be a little bit longer – maybe a little bit shorter – but let’s say roughly every seven days.’

So what’s going on? In order to understand what is happening, it’s important to follow four steps when looking at Covid case numbers as reported each day:

1. Work out what time frame you want to look at: you can either choose the date the specimen was taken (‘specimen date’) or the date the results were reported.