Sorry @sbfnk I don't see how this analysis takes us forwards. We need to know the +ve predictive value, not specificity, and we cannot get that without verifying LFT+ves with PCR. Your analysis only tells us that the specificity must be =< observed total +ves, not by how much.
https://twitter.com/sbfnk/status/1367502877946765317
Last week total +ves in staff and students was 0.07%, so we know specificity was >=99.93% (we don't need to do any maths to conclude that). So we know it is between 99.93% and 100% but have absolutely no clear where. Without that knowledge we cannot compute the PPV.
If it was 99.93% that means none of the +ves were real cases. If it was 100% then all +ves would be real cases. So we can't tell whether the PPV was 0% or 100% or where it is inbetween. No maths will solve it, only PCR verification.
This analysis cannot help @SMHopkins or @PHE_uk or anybody else have any confidence that the mass screening in schools is doing anything useful at all - hence the desperate pleas to verify with PCR. Now the +ve rate is so low the proportion getting a correct test result is lower
More than happy to discuss (email me) if you've think I've got it wrong or want to talk through my concerns.
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