Coronavirus

[Witton Park]

Higher %s of false negatives as opposed to false positives doesn't necessarily lead to under-recording when the virus level is low because it is a higher % of a lower number.

If FNs are 10% and FPs are 1% of tests.

Test 100000 people randomly you should get around 100-200 positives at our recent levels. Say 100.

Of those 100, you end up getting only 90 positives because of the 10% rate of FNs so that under represents.

The other 99900, they will have mostly had correct negative results, but 999 will have had false positive results.

So whilst 100 had it, 1089 are identified.

NB. This is based on an illustration I read a month or so back from Heneghan. I'm not claiming it as my own work, but I've applied those figures to illustrate the problem.
Hope my maths are correct

It reminds me of a similar anomaly highlighted last Spring in the calculation of the R number where a total drop in numbers can be presented as an increase in R.

[Dave_Mole]

Had you been testing in the public in March, any that you tested would almost certainly have been live positive tests as no one had hardly been exposed enough to have virus fragments.

Speculation.
The study by Oxford found that: "PCR results per se are unlikely to predict viral culture from human samples. Insufficient attention may have been paid how PCR results relate to disease. The relation with infectiousness is unclear and more data are needed on this".
The study was concerned with infectiousness, rather than presence/abscence of the virus and the Lancet argued that: "the findings in this study should not be used to conclude prolonged viral shedding or provide rationale to amend isolation policies".

You might also want to think about your sentence construction:

1 month later, you can still find the lice in some of the kids - usually dead ones -

[Muddy Retriever]

No test is 100%. One of the main issues with PCR is false negatives, so it's likely to under-record than over. Perhaps by 3-5%.


This is a completely false analogy.
Rainfall is not an infectious disease. The point of monitoring rainfall is to observe an overall pattern. The point of PCR tests (in Pillar 1) is to diagnose a disease. Pillars 2-4 are to determine the overall patterning, but have not been given sufficient resources in order to function at an acceptable level.
People can't even get Pillar 1 tests at the moment.

It is both Pillar 1 and Pillar 2 that are used for detecting new cases. Pillar 1 testing is performed in clinical settings like hospitals. Pillar 2 is performed by other organisations for the wider community, so this includes the drive through sites. It didn't exist to begin with but is now where the overwhelming majority of tests are carried out.

Pillar 3 is antibody testing and Pillar 4 is surveillance testing.

See here, click on "About" and scroll down.
https://coronavirus.data.gov.uk/testing

[Muddy Retriever]

Higher %s of false negatives as opposed to false positives doesn't necessarily lead to under-recording when the virus level is low because it is a higher % of a lower number.

If FNs are 10% and FPs are 1% of tests.

Test 100000 people randomly you should get around 100-200 positives at our recent levels. Say 100.

Of those 100, you end up getting only 90 positives because of the 10% rate of FNs so that under represents.

The other 99900, they will have mostly had correct negative results, but 999 will have had false positive results.

So whilst 100 had it, 1089 are identified.

NB. This is based on an illustration I read a month or so back from Heneghan. I'm not claiming it as my own work, but I've applied those figures to illustrate the problem.
Hope my maths are correct

It reminds me of a similar anomaly highlighted last Spring in the calculation of the R number where a total drop in numbers can be presented as an increase in R.

The problem of false positives is amplified by mass testing. It was mentioned in the Times article that Fellbeast linked to the other day. I have run out of my "free" articles so I cannot look back to see precisely what was said. However I think the gist was that if you were to test the whole population regularly as proposed by "Operation Moonshot" and the false positive rate was 1% then you would get over 600,000 people incorrectly told they had the virus and wrongly having to self isolate. So while superficially mass testing seems like a good idea, in reality it wouldn't be, especially if it costs £100 billion as some have suggested.

[Dave_Mole]

It is both Pillar 1 and Pillar 2 that are used for detecting new cases.

Sorry, yes, my mistake.
The point still stands regarding the surveillance side of things being tiny.
And the analogy with rainfall being pants.
But, as you suggest, there might be other reasons why this might be problematical. Is 600,000 false positives a "price worth paying" if we were to see a return to March levels of virus? Is that a likely scenario? So many uncertainties!!

[Mossdog]

"Reports from the cabinet subgrouping in charge of Covid policy suggest that the new ‘rule of six’ was chosen instead of eight not for epidemiological reasons, but for purposes of “messaging clarity”. It was thought that, since the number six was already out there, it should be retained for simplicity’s sake; eight would only complicate things. And so the lives of England’s 55 million citizens are to be drastically altered “for the forseeable future” according to the principles of campaign science".

Interesting article in Unherd today regarding England's handling of CV

https://unherd.com/2020/09/matt-hanc...nd=1&tl_groups[0]=18743&tl_period_type=3

"So what would good leadership look like during this period? ...It would involve respecting, but putting firm boundaries around, the contributions of scientists; being clear what your bigger-picture values are and how society should best mitigate the risks without being overwhelmed by them; admitting what you don’t know and yes, offering reassurance even when you’re not sure. Sadly, examples of this combination are hard to come by, and the Covid-19 era is a lot more frightening for that absence."

[Graham Breeze]

"Reports from the cabinet subgrouping in charge of Covid policy suggest that the new ‘rule of six’ was chosen instead of eight not for epidemiological reasons, but for purposes of “messaging clarity”.

So not five fingers + myself ?

[Mossdog]

So not five fingers + myself ?

Steady on GB! You're not making crude allusions similar to the meaning behind this one-hit wonder are you?

https://www.youtube.com/watch?v=IWWwM2wwMww

[Graham Breeze]

Steady on GB! You're not making crude allusions similar to the meaning behind this one-hit wonder are you?

https://www.youtube.com/watch?v=IWWwM2wwMww

Well - that has prompted some research.

[Dave_Mole]

Me, myself and I.
Twice.