Coronavirus

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Originally Posted by noel

Clearly false negatives are more alarming, because it results in many people who have covid and are infectious being told they're fine and to go back to work/school etc.

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I've thought that myself too. In some cases people are tested again. Look half way down this under the heading The Number of People Tested. One of the reasons it gives for people being tested more than once is:

"individuals with early symptoms who test negative, develop more symptoms and are retested and found to be positive"

https://www.gov.uk/government/public...thodology-note

But I imagine there will be many others who aren't tested again. If everybody was tested twice then the chances of a false negative would be only 4% assuming the false negative rate of one test is 20%. But I would think there is not the capacity to do that at the moment.

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Originally Posted by 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 :D

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.

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Noel - just for you in case you didn't see it previously.

and with respect to the heightened rates, the issue is still there. Just the numbers are adjusted.

Sure in the event of a higher public infeciton rate the effect of the False positive would be lessened, but it would still be there and then you also get to the greater concern that we would have a large number of false netagtives that go out in to the big wide world thinking they are OK unless they have symptoms.

Eiether way it needs sorting and at the moment it seems it hasn't been.

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Originally Posted by Muddy Retriever

I've thought that myself too. In some cases people are tested again. Look half way down this under the heading The Number of People Tested. One of the reasons it gives for people being tested more than once is:

"individuals with early symptoms who test negative, develop more symptoms and are retested and found to be positive"

https://www.gov.uk/government/public...thodology-note

But I imagine there will be many others who aren't tested again. If everybody was tested twice then the chances of a false negative would be only 4% assuming the false negative rate of one test is 20%. But I would think there is not the capacity to do that at the moment.

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On your link.

This means that currently all positive cases identified by pillar 4 surveillance studies (for antigen testing) are captured under pillar 1 or 2.

Which seems to indicate that someone with anti-bodies is a positive case. Even if they had been infectious a few weeks or months earlier.

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Originally Posted by Witton Park

On your link.

This means that currently all positive cases identified by pillar 4 surveillance studies (for antigen testing) are captured under pillar 1 or 2.

Which seems to indicate that someone with anti-bodies is a positive case. Even if they had been infectious a few weeks or months earlier.

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Yesterday Pillar 4 tests accounted for 1% of total tests.

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Originally Posted by Witton Park

Noel - just for you in case you didn't see it previously.

Sure in the event of a higher public infeciton rate the effect of the False positive would be lessened

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Thanks WP, I hadn't read through a lot of the earlier posts on this thread.

And yes, that's a good point that I hadn't previously considered: When the actual infection rate is very low, the issue of false positives is not dwarfed so much by the effect of the false negatives. So in essence it might be a small red herring, but it's in a small pond.

I'm not sure where things will go with "sorting" it. There are obvious steps to improve specificity and sensitivity (eg, increasing the number of samples per person or even tests per person), but this would have a detrimental effect on the number of tests than can be carried out per day. So at the moment it's a bit of a balancing act.

Also, although we're envisaging a worst-case situation where all the false negatives immediately go and mingle in society, in fact many of these people will probably stay at home because they'll be ill. It's only asymptomatic or mildly symptomatic people who will return to their normal routine.

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Originally Posted by noel

Thanks WP, I hadn't read through a lot of the earlier posts on this thread.

And yes, that's a good point that I hadn't previously considered: When the actual infection rate is very low, the issue of false positives is not dwarfed so much by the effect of the false negatives. So in essence it might be a small red herring, but it's in a small pond.

I'm not sure where things will go with "sorting" it. There are obvious steps to improve specificity and sensitivity (eg, increasing the number of samples per person or even tests per person), but this would have a detrimental effect on the number of tests than can be carried out per day. So at the moment it's a bit of a balancing act.

Also, although we're envisaging a worst-case situation where all the false negatives immediately go and mingle in society, in fact many of these people will probably stay at home because they'll be ill. It's only asymptomatic or mildly symptomatic people who will return to their normal routine.

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the extreme end of false positives is that if you test a cobvid free are say like the Scilly Isle with corca 2000 folk, a FP rate of 0.5% would give you 10 positives, trigger a load of contact tracings, quarantine and give them a 500/100,0000 population rate that would propel them to the top of the national rankings.

Article headed: Our Covid strategy is based on faulty assumptions? Claiming

"The Government’s strategy of introducing restrictive measures, both local and national, whenever positive tests start to rise is based on three critical assumptions:

1. Once positive tests start to rise, they will inevitably increase to catastrophic levels without intervention

2. Big increases in positive tests will eventually lead to high numbers of hospitalisations and deaths

3. The restrictions will be effective in cutting infections

Given the astronomical costs of the restrictions, it is hard to see how they can be justified if any one of these three assumptions does not hold. The evidence so far suggests that, in fact, none of them may be valid".

https://unherd.com/thepost/governmen...y-assumptions/

There's does seem to be an inexplicably high degree of variations across the Europe, irrespective of the type and level of any restrictions imposed.

https://www.bbc.co.uk/news/world-europe-54391482

All very confusing :confused:

There was a gentleman on the wireless this morning talking about just that. Basically he said Sweden had achieved the same degree of actual social distancing as we have, and that if we'd've introduced the same measures as Sweden did, it wouldn't have had the same results as they achieved.

You can draw your own conclusions regarding what that says about our or their society.

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Originally Posted by noel

There was a gentleman on the wireless this morning talking about just that. Basically he said Sweden had achieved the same degree of actual social distancing as we have, and that if we'd've introduced the same measures as Sweden did, it wouldn't have had the same results as they achieved.

You can draw your own conclusions regarding what that says about our or their society.

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Nail on the head: everywhere is different in many different ways which include (but are not limited to) population density, willingness to wear masks, what is deemed socially appropriate and general common sense....

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Originally Posted by Dave_Mole

Nail on the head: everywhere is different in many different ways which include (but are not limited to) population density, willingness to wear masks, what is deemed socially appropriate and general common sense....

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And didn't the chap talking about Sweden say that people there trusted what their government said more than people in the UK? Obviously I am at a complete loss to explain why anyone here wouldn't trust everything that our Government says . . . :p