Funnily enough despite all the news coverage I've not previously seen how it's calculated.
R=(M1+M2+M3+M4)/(M5+M6+M7+M8)
M - daily measure of Covid19
1 to 8 represent day of measurement - 1 day earlier, 2 days earlier, 3 days earlier...
Basically it's how the measure changes on a timescale representing the 4 days when somebody might be infectious. Choice of 4 days and averaging over 4 days presumably not written in stone as you would use months or years for something much longer term like HIV.
I interpret the formula as a 4 day timescale for disease and 4 day averaging rather than 8 day disease timescale.
If you use too short a timescale you underestimate changes making R closer to 1, if you use too long a timescale you overestimate change and make R further from 1.
If the timescale too short/long it will still correctly give R=1 when the measure is static.
Saw this on German news, starting at 02:35 in the video below. The graphics at 03:22 might be understandable without understanding German.
http://www.tagesschau.de/multimedia/sendung/ts-37053.html
I do not know what formula the UK uses to calculate R or which of the multiple possible measures it uses. I don't know if the UK is testing a representative random sample.
So far I've not said what M is.
I think the official calculation in Germany comes from Robert Koch Institut and I think the measure I've written as M is hospital admissions (I didn't get that from the video, a German contributor to UKC mentioned it in passing).
The actual German formula is
R=(M4+M5+M6+M7)/(M8+M9+M10+M11)
because the latest 3 measures M1, M2 & M3 are provisional and subject to change (my understanding of the video).
Anyway, if you have the data (eg URL below for example) you can calculate R with Excel etc for yourself.
https://www.gov.uk/government/publications/slides-to-accompany-coronavirus-...
Which M for R=(M1+M2+M3+M4)/(M5+M6+M7+M8)?
I calculated for UK daily deaths which I think lags ON AVERAGE about 3 weeks behind day of infection plus a bit for use of 7 rolling sum, this is currently about R=0.86 reflecting what was happening 3-4 weeks ago.
For more up to date information I calculated R for Hospital admissions (E+W) which I think lags ON AVERAGE about 10 days behind day of infection. This is currently R=0.89 and fluctuates more, perhaps needs a 7 day average too.
I calculated R for People in hospital for English regions & Scotland, Wales, NI. I think that is appropriate but I may be wrong. It has multiple timescales as it depends on admissions, discharges & deaths. I think R=0.88 to 0.93 for all of them now.
I've taken then government spreadsheet (yesterday) and calculated R using the latest data. The columns I've added are red text, I've not changed the other data (unless I miss-typed somewhere) but you can download that for yourself anyway.
You can see R was much higher before the lockdown.
https://gofile.io/d/VqQtqQ <-- my version of government spreadsheet, my additions to calculate R in red.
PS my background is NOT medical related
Post edited at 14:28