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expert reaction to latest R number and growth rates estimates, published by the government

The government have published the latest estimates for the COVID-19 R number and growth rates.


Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“The weekly updates from SAGE to the ranges of estimates for the R number and the growth rate for England and for the NHS regions of England have been published. It’s not entirely clear how much information value these estimates have, given that infection rates generally are low and (according to the ONS Covid-19 infection survey and the data on the dashboard at still falling. At this stage in the pandemic, the pattern of infection is likely to be dominated by outbreaks in geographical areas that are much smaller than the large NHS regions, let alone the whole country. Also, the relationships from last year between infection levels on one hand, and serious illness, hospitalisation and deaths on the other, have been very radically changed by the continuing success of the vaccination programme.

“But, that said, what changes have been made in the ranges? The range for R for the whole of England this week is from 0.8 to 1.0. That’s the same as it was the week before last, and indeed for two weeks before that – but its lower end has changed from last week, when the range ran from 0.7 to 1.0. The narrowed range means that SAGE are a little more certain about where the R value lies, and if anything, it may mean that R has increased a little. But that isn’t definite. For example, the R number could have been 0.9 last week and also this week, since that’s in both ranges. SAGE are still reasonably certain that R is below the level of 1.0, above which new infections would be increasing.

“The range for the growth rate for England as a whole goes from -5% to -1% per day. This means that SAGE estimate the number of new infections is falling by between 5% and 1% each day. If the growth rate is -5%, that means that, for every 100 infections today, there will be 95 tomorrow. And if that went on for some time, the number of infections would halve in about two weeks. A growth rate of -1% would mean 99 new cases tomorrow for every 100 today, so that infections would halve in about ten weeks if that carried on for a time. More likely, the growth rate will be somewhere between these two ends of the range. If it were -3% per day, say, that would correspond to infections halving in around three weeks. Last week’s growth rate range for England was -6% to -1% per day, so, like the R number, a small adjustment has been made at the lower end. SAGE still seem to be pretty confident that new infections are continuing to fall, because the interval does not go above 0, but they are rather uncertain about how fast it’s falling.

“In a way it’s barely worth interpreting that R and growth rate ranged for the NHS regions, because for six out of the seven regions, SAGE state that ‘Particular care should be taken when interpreting these estimates, as they are based on low numbers of cases or deaths and/or dominated by clustered outbreaks. They should not be treated as robust enough to inform policy decisions alone.’ This relates to my previous point that, at this stage in the pandemic, the picture is likely to be dominated by outbreaks in relatively small areas. Several of the ranges have changed from last week, generally by not very much. It’s worth mentioning that the R ranges for three regions (East of England, London, South West) have their upper end above 1.0. This means that SAGE consider it possible that numbers of infections may tend to increase there – though in each case more of the range is below 1.0 than above 1.0, so that it’s more likely that R is really below 1 in those regions. In two of those regions (East of England and London, but not South West) the upper end of the range for growth rate is above 0, again indicating there’s a possibility that infections are growing there. Overall that’s a slightly more pessimistic position on the regions, particularly those in the south of England, than last week. But the caution that SAGE give about using these estimates, and also the fact that today’s figures from the ONS Covid-19 infection survey don’t give any clear evidence of increases in these regions, mean that I’m not particularly concerned.”



All our previous output on this subject can be seen at this weblink:



Declared interests

Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee.  I am also a member of the Public Data Advisory Group, which provides expert advice to the Cabinet Office on aspects of public understanding of data during the pandemic.  My quote above is in my capacity as an independent professional statistician.”

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