One thing which might be a factor is the number of tests that were spoiled. You have to process a PCR test within a certain time or else the result is neither positive nor negative, but unknown -- you need to test again. I don't know how significant spoilage is, or what gets done with the spoiled tests, but it could be a significant factor.
Yes indeed, spoiled tests and indeterminate test results make things tricky. On top of that if a person tests positive and negative within the same week then only the positive test result is recorded. Nowhere can you obtain daily totals of positive results, negative results, spoiled results and indeterminate results and nor can you obtain full counts of people who repeat test each week - hence all the fiddling!
The section headed "From 15 October 2020, the methodology for people tested and people testing positive has changed" is where the juicy bits are found.
One thing which might be a factor is the number of tests that were spoiled. You have to process a PCR test within a certain time or else the result is neither positive nor negative, but unknown -- you need to test again. I don't know how significant spoilage is, or what gets done with the spoiled tests, but it could be a significant factor.
Yes indeed, spoiled tests and indeterminate test results make things tricky. On top of that if a person tests positive and negative within the same week then only the positive test result is recorded. Nowhere can you obtain daily totals of positive results, negative results, spoiled results and indeterminate results and nor can you obtain full counts of people who repeat test each week - hence all the fiddling!
A useful link for folk interested in the methodology...
https://www.gov.uk/government/publications/nhs-test-and-trace-statistics-england-methodology/nhs-test-and-trace-statistics-england-methodology
The section headed "From 15 October 2020, the methodology for people tested and people testing positive has changed" is where the juicy bits are found.
and then there are all the repeat positive tests where people are desperately testing to free themselves from the spare room!
Quite!