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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:

“This is the second week running that SAGE have not agreed overall figures for the R number and the growth rate for the whole of the UK, and in fact they now say that such figures “will no longer be produced”.  I think that’s reasonable – the way that the pandemic is moving is different in different UK countries, and the decisions on control measures are mostly made by the relevant governments and are not the same in the different countries, so combining data for all four countries doesn’t make much sense now, and arguably never made all that much sense.  Even just in England, there are considerable differences between the trends in the pandemic in different places.  Indeed, this week, SAGE have flagged the R and growth rate figures for four of the seven NHS regions with an asterisk, meaning that they are based on low numbers of cases or deaths and/or are dominated by clustered outbreaks.  This would mean that the estimate for the whole region may not be very reliable, and may not be a very good guide to what to do about infections even in that single region.  We’ve become pretty obsessed with R numbers over the past year or so, and this is a clear warning that they just don’t give a very clear picture of what’s going on or what to do about it, at least not at the geographical level of whole UK countries or even regions.  That’s even truer than it was, now that vaccination is radically changing the way that infections might lead to serious illness or death.  Personally, I find growth rates generally more informative than R numbers anyway, because they are easier to estimate and easier to think of in terms of trends in infections – but they can also disguise big and potentially important differences between UK countries or even between different parts of the same English region.

“That said, what do this week’s numbers look like?  The R range for the whole of England is 0.8 to 1.0, which is exactly the same as it was last week and the week before.  It doesn’t go above 1, so that SAGE are saying they believe that infections probably aren’t growing.  The fact that its upper end is exactly at 1.0 means that SAGE believe it’s possible that infections are static, but apart from this the range is all below 1 so SAGE must think it’s most likely that infections are falling.  The growth rate range for England is -4% per day to 0 per day, which again is exactly the same as last week and the week before.  A growth rate of -4% actually means that the number of new infections is going down, and that, for every 100 new infections today, there will be 96 tomorrow.  If that rate continues, the number of new infections would halve in about two and a half weeks.  A growth rate of 0 means that infections remain about the same from one day to another.  So again, SAGE are saying that probably the number of infections is falling from day to day, but it could just about be static if the true growth rate is at the top of their range.

“In broad terms this looks slightly different from what was estimated in the ONS infection survey, published earlier today.  The incidence estimates there, that is, the estimates of the daily rate of new infections, were slowly increasing for England, not falling.  However, those ONS incidence estimates only go up to the week ending 27 March, and the data on new confirmed cases from the dashboard at do show signs of further decrease after that.  But, broadly, all these sources are saying roughly the same thing, which is that infection rates in England really aren’t changing much in either direction.

“Most of the R and growth rate ranges for the English NHS regions are either the same as last week (and often the same as the week before that as well), or have changed only very slightly.  None of the regional R ranges go above 1.0, though all but one of them do have 1.0 as their upper end.  Likewise, none of the regional growth rate ranges goes above 0.  The biggest change in the regional ranges, compared to the previous week, is for the South East, where the growth rate range was -5% to -1% last week and is -7% to -2% this week – that is, SAGE are saying that the daily case numbers could be falling slightly faster than before.  However, the South East is one of the regions flagged as having estimates that are difficult to interpret, so I really don’t think I should try to read anything into that change – things are too uncertain if we consider that region as a whole.  That isn’t in itself any cause for concern, though – it’s just how things are at this stage in the pandemic.”



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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