to explain why testing lots of asymptomatic people everyday is a bad idea

[chomalungma]

They want to do lots of testing on asymptomatic people everyday.
Aim is to see if people are positive, and if so, then those people have to self isolate.

Issue is: There will be many people who will have to self isolate who don't need to because of false positives

All to do with how specific a test is, how sensitive it is, and the prevalence of disease in the population you are screening.

Let's say the levels of disease are 1 in 10,000

So in 1 million people, 100 people will have the disease

And 999,900 people won't have the disease.

They get a really good test; It detects disease in 99% of the people who have it.

So in the 'ill' population, we have 99 positives and 1 negative

The test is also very specific. Say 99.5% specific. That means in the 999,900 people who don't have the disease, we will get 994,900 people who test negative and 5000 who test positive (False positives)

So we have 5099 positive tests.
And 99 of them are actual positives

So 98% of the positive results are false positves

(this is based on a level of 1 in 10,000 and those figures for specificity and sensitivity) Obviously a test needs to have really good specificity and sensitivity and it needs to be done on a population with a relatively high chance of having the disease.

[quote Malteserdiet]@chomalungma
Nothing else to add and totally agree with you. But also wanted to say..... you’re awesome

Please can you replace Chris Twitty asap?[/quote]
I REALLY want to have the chance to talk to Johnson about his use of statistics.

For anyone else who wants to know what specificity means;
The specificity of a test is its ability to designate an individual who does not have a disease as negative. A highly specific test means that there are few false positive results.

Thank you for posting this OP. You are clearly qualified to have an educated opinion that is not based on Google. Please carry on posting to educate the rest of us. I have BSc Biology but different specialisms.

Really interesting post OP, I wish you could replace BoJo (and I bloody voted for him)

@chomalungma

I am off to ask Google

Let's say you have 100 people who do NOT have the disease.

If the test was 100% specific, then all of them would test negative.
99% specific would mean that 99 would be negative and 1 would be positive.

Thank you!

True, false positives are a problem but what is the alternative?
Under a general lockdown aren’t we assuming that everyone is potentially contagious. So even with a highish rate of false positives it’s still a better option than trying to control it through lockdown.

So, are you able to work out what the actual positive figure would be when the daily total of new cases is declared as 2919? Or do you need more stats? I’d be interested to understand what the real daily figure is more likely to be but have never had a maths brain.

In this case, regarding the disease spread, aren’t false positives better than false negatives? Just in terms of more people in isolation?

Sorry, if I’m not quite catching on but I’m sure there are people self testing at home who haven’t done it right, who are actually positive, and going back out too soon.

In this case, regarding the disease spread, aren’t false positives better than false negatives? Just in terms of more people in isolation

I think if we move to mass testing as has been proposed, then it's going to be an issue.

Do we want a lot of people self isolating who are actually disease free?

False positives in mass testing - even if it was 99.9%, in the example in the OP there would be 1099 positive tests, with only 99 of them being genuine.

Over 66 million people, that would be 72,000 ish positive tests. With only 6500 being genuine. That's 65,000 people having to isolate

when they don't need to. Plus MSM shouting about how many people are positive when the reality is VERY diffeeent.

I wonder if this has been clearly explained to Boris Johnson et al

And if he actually understands it?

Thanks for posting this, I totally agree. Hasn't Carl Heneghan done some research on this recently?

Qualifications: none apart from being an ITU nurse and having some training in reading/critiqueing medical research/stats. Does that count?

It's a simple cost-benefit analysis. Knoing how many asymtomatic people there are is important. Yes there is a 'cost' in terms of false positives having to self isolate unnecessarily but that's a necessary evil. Hopefully in future there'll be even more reliable tests and a faster turn around so assuming the false positives aren't correlated we could simply test twice for each person and the number of false positives would be very small.

Do we want a lot of people self isolating who are actually disease free?

Again, isn’t this exactly what we have with a general lockdown. Surely even if testing doesn’t creates some false positives it’s better than everyone locking down again?

@NothingIsWrong

when they don't need to. Plus MSM shouting about how many people are positive when the reality is VERY diffeeent.

The RT-PCR tests are always going to be better than any home tests or pregnancy test equivalent.

I have no idea about the false positive rare for the RT-PCR tests that re being done on a population who suspect they have Covid-19

I am just talking about the proposed Moonshot testing

The other problem comes when they start with Zero Covid. Unless the test is 100%, there will always be detected case. Always

@chomalungma Boris Johnson is a moron but there's no way he doesn't understand this because it's very very basic. Even a very average A-level student in maths could work this out. Boris might well be less mathematically literate than an Alevel student but it will have been explained to him. You certainly haven't thought of something that wasn't very very obvious to the people working this stuff out.

(This is actually very similar to a standard conditional probability question I ask A-level stats students).

Over 66 million people, that would be 72,000 ish positive tests. With only 6500 being genuine. That's 65,000 people having to isolate

But still better than the alternative of having lockdown and 66million isolating.

Do you think the Moonshot will be less specific then? The problem just multiplies up then...

Do you follow Jon Deeks on Twitter, OP?

The principle that the govt is allowed to widely infringe on our civil liberties as part of the COVID control measures is already well established. Most the populace are fearful & Fear is driving policy.

Folk may also wish to read this BMJ editorial.

Surely even if testing doesn’t creates some false positives it’s better than everyone locking down again

As has just been said - how many people do we lock down just to catch the ones who have it?

It is a cost-benefit analysis.

Or do we do much better contact tracing, rapid testing of contacts

False negatives, how likely are they in your scenario ?

@SherryPalmer

Over 66 million people, that would be 72,000 ish positive tests. With only 6500 being genuine. That's 65,000 people having to isolate

But still better than the alternative of having lockdown and 66million isolating.

All those 65,000 people will then be contact traced, with their school bubbles being sent home for 2 weeks and all their contacts having to isolate as well.