PHE seems to have changed the way it calculates positivity rates, 4 weeks after I pointed out they appeared to be using the same incomplete count of the number of people tested that's given in the Test & Trace report.
This massively and increasingly overstated positivity rates.
This massively and increasingly overstated positivity rates.
https://twitter.com/Dr_D_Robertson/status/1314243911448223746
The problem is they divided the number of people testing positive by the number of people tested for the first time.
Anyone who had ever been tested before was left out of the figure they were dividing by (unless they were positive for the first time).
Anyone who had ever been tested before was left out of the figure they were dividing by (unless they were positive for the first time).
https://twitter.com/_johnbye/status/1304320944983465984?s=19
Satellite centre tests (mostly repeat testing of care home staff and residents who have been tested before, and therefore don't count as new people tested) now make up almost half of all Pillar 2 tests.
You can see the impact this has by comparing tests done to "people tested" -
You can see the impact this has by comparing tests done to "people tested" -
Dividing by the number of people tested for the first time, which now excludes perhaps half of all people tested each week, effectively doubles the positivity rate if you calculate it this way.
Positivity rates ARE going up, but nowhere near as fast as published figures suggest.
Positivity rates ARE going up, but nowhere near as fast as published figures suggest.
If you extend this to the extreme case that EVERYONE has been tested before, only people who tested positive for the first time would be counted, both as people tested positive and people tested.
Which would generate a 100% positivity rate!
The change in methodology fixes this.
Which would generate a 100% positivity rate!
The change in methodology fixes this.
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