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Frequently Asked Questions

Rt The Effective Reproduction Number

Rt is the average number of infections a single infected person passes on to other people. It takes the Basic Reproduction number R0 and for a given time period t, it multiplies it against the portion of the population that remains susceptible. If the number is greater than 1 then the virus will spread, below 1 and it will contract.

How is it calculated?

We use coronavirus case data and where it is reliable and readily available, the test positivity rate calculated from the number of positive tests and the total number of tests along with a confidence factor dependent upon sample size to produce Rt. At all times we aim to be driven by the data rather than build in assumptions. In scenarios where data sample sizes are small the Rt value can experience wilder swings and we encourage the user to zoom out to a larger view at regional or national level in such instances.

Why is the UK data at least 4 days old?

Data for the UK is taken from the Department for Health and Social Care and the devolved administrations of the UK on a daily basis. The data is presented by specimen date (the date when the sample was taken from the person being tested). This data is accurately registered to its actual date of occurrence and is therefore more accurate than by Reporting date. The more recent days are subject to such heavy revision that it is misleading to publish them until at least four days have passed. Cases are allocated to the person's area of residence.

Where a date is published alongside Rt, does that apply to the case data also?

Yes, the UK Nations, Regions, Counties and US States all publish a date that all the data on that page refers to.

UK Case Rate Comparison currently shows English regions.

Apologies to Welsh, Scottish and Northern Irish users. The intention is for all counties to be presented on this single page in the near future. Although the data sources are the same, we are currently using two different access methods with the Welsh and Scottish data being retrieved in near real-time. When the approach is standardised then all UK regions/counties will be included.

Data Sources

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