I was half expecting a machine that would label all the presents as 'not a phone'. I mean, you could bump up your accuracy to 95% with that!

@TheZotmeister

4 жыл бұрын

^ Underrated comment.

@DukeBG

4 жыл бұрын

that's why a single number for accuracy of the test is misleading and two numbers should be given instead. Matt's machine had 20% chance of an error of 1st kind (false positive) and 20% chance of an error of 2nd kind (false negative). The "better" 95% accurate test of "everything is not a phone" would really be 0% error of the 1st kind and 100% error of the 2nd kind.

@mvmlego1212

4 жыл бұрын

True. That's why, when evaluating binary classification systems, we usually don't use accuracy as a metric. Instead, we use precision (percentage of selected items that are relevant), recall (the percentage of relevant items that are selected), and a F1-score (which is a combination of precision and recall). en.wikipedia.org/wiki/Precision_and_recall

@Maninawig

4 жыл бұрын

My math could be wrong, but it would still be 80% accurate. Out of the boxes containing phones, it can accurately predict it 80% of the time, so it will accurately predict 4 out of the 5 phones. However, the accuracy also applies to the ones whom are not phones (as it must determine there are no phones there either) so it will be 80% accurate meaning it will accurately detect that 76 of the 95 boxes aren't phones and label the other 19 as phones. They came up with 17% at the end because the machine would claim a total of 23 boxes would be labeled as phones. Out of that, 4 are correct, or 17.39(...)% of the newly wrapped boxes are phones.

@DjLepLaz

4 жыл бұрын

@@TheZotmeister literally the highest rated comment

I would like to reiterate that this machine is 80% accurate.

@rodrigoserafim8834

4 жыл бұрын

I think Matt was only 80% accurate on that statement.

@NicGarner

4 жыл бұрын

I would like to reiterate that this comment is 80% accurate.

@doublespoonco

4 жыл бұрын

I really didnt understand. How acurrate is it?

@iangabriel5536

4 жыл бұрын

@@doublespoonco, 80%!

@damienw4958

4 жыл бұрын

Huh, must have missed that bit...

4:32 My machine *slaps roof* is 80% accurate.

@RedRad1990

4 жыл бұрын

This bad boy can fit so many f***in phones inside

@user-pw5do6tu7i

4 жыл бұрын

Solid joke.

Don't take 50% of the credit. Take 80% of the credit.

@woutervanr

4 жыл бұрын

So 17%?

@NuclearTopSpot

4 жыл бұрын

Can I have the 20% of not-a-credit then? ʷᵃᶦᵗ ᵗʰᵃᵗˢ ⁿᵒᵗ ʰᵒʷ ᶦᵗ ʷᵒʳᵏˢ ᶦˢ ᶦᵗˀ

I came for Matt Parker and his machine, but getting a bonus of Hannah Fry as well is like my own perfect Christmas Present.

@tncorgi92

4 жыл бұрын

80% of the video was Hannah.

@klaxoncow

2 жыл бұрын

I came for Hannah Fry. I'm tolerating that Matt Parker is involved.

Better title: Parker Machine

@standupmaths

4 жыл бұрын

FINE. Updated.

@kishtarn555

4 жыл бұрын

@@standupmaths Yey!!!! We did it boys

@Niyudi

4 жыл бұрын

@@standupmaths only you would embrace that joke so gracefully hahaha

@alexsantee

4 жыл бұрын

For those who came later, the tittle was "Matt's machine which is 80% accurate."

@LuccaAce

4 жыл бұрын

There are currently 80 likes on this comment, by the way.

I want ‘It’s 80% accurate’ merch. And you only get sent the right merch 80% of the time.

@sword7166

4 жыл бұрын

Is it a phone 17% of the time?

@sixstringedthing

4 жыл бұрын

It's a T-shirt with the legend "I am a T-shirt" on the front. Except that it randomly changes to read "I am an off-duty Czechoslovakian traffic warden" 20% of the time. (Internet points to anyone who gets the reference).

@somethingelse4424

2 жыл бұрын

80%Acbutate

@remicou8420

Жыл бұрын

but you’d have to send a shirt to 20% of people who didn’t ask for one too

@TaijanDean

4 ай бұрын

​@@sixstringedthingYou Smeeeeeeg Heeeeaaaad!

I played the video 4 to 5 times just to see Matt's expression each time he repeated, "80% accurate!". He managed to vary his expression in subtle ways each time, not to mention the complacent little nod after each iteration.

These two make fantastic presenters for children's television. High energy, speaking to their audience like real people, genuinely humourous premises. I hope these two team up for more or make this sort of thing a regular side hustle.

I can already see people providing 80% accurate solutions to the next Matt Parker's Math Puzzle. Jokes aside though, what a neat way to explain this at first sight unintuitive part of probability theory!

@MithicSpirit

4 жыл бұрын

Not completely sure about this but it feels very related to Bayes' Theorem. EDIT: grammar

@k_tess

4 жыл бұрын

Parker math is math that is 80% accurate, or 80% to the goal.

@k_tess

4 жыл бұрын

For instance choose any positive integer n. Such that it forms a regular n-gon. If those n-gons are congruent, and can be used as faces to form a solid that is 80% enclosed, then the n-hedron constructed, is a Parker Platonic Solid.

I like the part where he says the machine is 80% accurate.

@dimitrispapadakis2122

4 жыл бұрын

When did he say that?

@shnob4916

4 жыл бұрын

Sorry, when? I didn't catch that one

@RobertHartleyGM

4 жыл бұрын

timestamp?

@gasdive

4 жыл бұрын

He's only saying that for 17% of the video.

@CaptainSpock1701

4 жыл бұрын

@@gasdive Your 17% of the video is 80% accurate!

Parker Salesman: *slaps top of machine* This bad boy is 80% accurate

Never have I seen Bayes theorem explained so simply and with such fun. Call me a skeptic but I suspect the 5 mobile phones were fake news - got to watch those budgets.

@gildedbear5355

4 жыл бұрын

in reality there were 5 brand new mobile phones in the boxes but they were in the /red/ boxes. when the boxes got switched all of the actual phones got dumped on the ground.

@Vulcapyro

4 жыл бұрын

@@gildedbear5355 Wow, a 0.032% occurrence, bet they didn't see that coming at all

@SlimThrull

4 жыл бұрын

I'd say that the idea of the phones all being fake is about 80% accurate.

@henryambrose8607

4 жыл бұрын

Probably. Not only for budget reasons, but because having the person at the end pick out an actual phone would defeat the point of the demonstration.

@gasdive

4 жыл бұрын

Phones are cheaper than socks.

Well, I mean, the machine improved the girl's chances of finding a smartphone by 247.8%, so it wasn't a total loss... Or something. ;p

80% of the time, I'm 80% right.

@zoltanposfai3451

4 жыл бұрын

@@epsi What if 80% of the time he's 80% accorate, and 20% of the time he's 100% accurate? :P

@gyroninjamodder

4 жыл бұрын

@@epsi He could be 1% right and his comment would be a true statement.

@londonreturns

4 жыл бұрын

your statement works 60% of time everytime.

@aliceanderson5154

4 жыл бұрын

@@epsi -- so I'm 14 percent more reliable than a coin flip? This gets 7% more confusing as time goes by.

@kunalkashelani585

4 жыл бұрын

So, you are atleast 64% right and atmost 84% right(considering that this time you are right!)✌️

Matt's embracing the true Parker spirit

This has got to be near-optimal for children's education. You've got 3 hours -- can we get 6 hours * 5 days * 40 weeks * 12 years - 3 hours = 14397 more, if we all work together?

another less-mentioned but equally important result of this sort of error: one of the positives ended up on the floor. In the video that means a smartphone, but with the current events that means a contagious case of the human malware that didn't get caught by screening. Testing helps, but is part of the equation, not the entire solution.

@olivier2553

4 жыл бұрын

That's why, depending on the problem, you try to optimize the false positive or the false negative, considering it is better to detect miss classify a non infected as being infected or better to miss detect an infected case.

@olmostgudinaf8100

4 жыл бұрын

That was *literally* the point of the video.

@massimookissed1023

4 жыл бұрын

3Blue1Brown vid modelling virus spreading, including testing & quarantine, and when that testing isn't perfect. kzread.info/dash/bejne/max1w7FrotbKedY.html (23mins)

@reydien1658

4 жыл бұрын

@@olmostgudinaf8100 I would say the main point of the demonstration was on False Positives, considering the notion of False Negatives was never even mentioned at all. They focused on the fact that even with 80% accuracy there were 19 false positives on the table, while the false negative was barely given a passing mention and never brought up again.

Ahh so it’s a Parker machine. This comment looks stupid since he changed the title lol.

@mikeuk1927

4 жыл бұрын

Anthony Flanders Not only is it a Parker machine. It's also made by Matt Parker. It means that it's a Parker Parker machine. Or simplifying Parker² machine. We've come a full circle to Parker square.

@pppfan103

4 жыл бұрын

Matt Parker: The King of Imperfection

@spawniscariot9756

4 жыл бұрын

@@mikeuk1927 Surely not a full circle 😜

@spawniscariot9756

4 жыл бұрын

@Hyasconi "Matt's machine which is 80% accurate"

@KrisPBacon69

4 жыл бұрын

@@mikeuk1927 you mean a parker circle to a parker square?

It's still a bit unclear, how accurate is the machine?

@jmbreche

4 жыл бұрын

80%

@reubenm.d.5218

4 жыл бұрын

Not very

@Richard-Swift

4 жыл бұрын

Somewhere between 75% and 85% accurate

@matthewryan4844

4 жыл бұрын

i'm 80% sure he mentioned that

@adivp7

4 жыл бұрын

The process (finding a phone) is now 17% accurate instead of just 5% because of the machine being 80% accurate

Please stop making things that sort of work or else the word "Parker" will be everywhere

@Anvilshock

4 жыл бұрын

Nice Parker comment.

@theajayyy

4 жыл бұрын

He changed the title!

@Maninawig

4 жыл бұрын

By this comment and the message of the video, all medical tests are Parker tests...

@randomdogdog

4 жыл бұрын

But it does work! It's 80% accurate!!

@dozenazer1811

4 жыл бұрын

Parker pens xd

Love the Christmas lectures usually, even more so with you and Hannah doing it

I love the name is "X-mas ray detector 0.80"

@olmostgudinaf8100

4 жыл бұрын

I wonder how many spotted that.

@patricktho6546

4 жыл бұрын

At our Uni (Würzburg, Germany) we have a physics christmas lecture in witch it's discussed if "Santa" can delivere them in time. Because the X-Rays were discovered here by Mr Röntgen, we use a real X-Ray-Machine to identify the presents :)

I watched all 3 progs when first broadcast, and the 80% accurate machine struck me then as an absolute highlight of brilliant & clear explanation. Bravo, both

I love the non-intuitive way that a low incidence rate can make the tests pretty much meaningless. But on a serious note, how often is the accuracy on these tests symmetric? In a lot of cases, you can adjust the sensitivity, to give more positives or more negatives. It would be interesting to look at the impact that could have , eg if your machine correctly flagged _every_ phone as a phone but incorrectly flagged 50% of non-phones as phones. I guess in different situations, it might be more beneficial to swing the bias one way or the other, depending on what you are looking for.

@paulh.9526

4 жыл бұрын

Indeed, tests are biased a bit for best accuracy at a specific incidence rate. Also, tests used for screening will be biased more towards the positive, because false positives can be checked, false negatives can be deadly.

@Pablo360able

3 жыл бұрын

And like that you've rediscovered the bias-variance tradeoff. It all depends on how you want to measure failure (what statisticians call the “error function”) and minimizing that. For instance, if false positives are worse than false negatives, you might construct a test that minimizes an error function which assigns a higher value to false positives than to false negatives.

@zvxcvxcz

2 жыл бұрын

In biophysics we are keenly aware of such issues and tend to use a different metric known as Mathew's Correlation Coefficient (MCC) rather than things like accuracy and sensitivity. MCC works very nicely even with extremely unbalanced data. Oh, and this Mathew is not Parker, lol. The machine learning folk are only catching on to this metric much more recently. MCC won't always be most appropriate, for instance if you're working on a problem where false positives have no consequences, but I think it is a better general use metric than any other, even if it may be less intuitive to derive and a bit more involved to calculate.

Omg you changed the title to "Parker Machine"

I remember this! It was a most effective way of demonstrating the principle. Well done, you and Hannah!

the RI channel really has some great videos. Long form but informative and thought provoking

I was very much looking forward to this video!

Watching the full lecture, the bit that comes after this one is completely relevant today.

What a fantastic clip and visualisation, well done.

Well, ain't that a Parker Square of a machine?

Already saw this on the RI channel, but I’m watching it again on here anyway.

woow I like how you guys deliver this with the children and all. nice show! good vibes!

From the beginning, I wanted to chuck the 23 positives back into the machine. Rounding down, the 19 junk and 4 wins becomes 3 of each. Stripping back the metaphor, this is not always possible. Perhaps something in the original sample was what led to the false positives and negatives. But if the fault was in the testing, then retesting will improve the results.

That's why I just run the "phones" back through the machine!

@arnhelmkrausson8445

4 жыл бұрын

That would only work if those were random false positives. What if there was a specific property that caused the false positives? You'd end up with the same 23 rewrapped boxes.

@patricktho6546

3 жыл бұрын

@@arnhelmkrausson8445 I shouldn't be a specific property to be a false positive, since this would be a systematic error and with propagation of uncertainty you ony take random errors in account. And since this is the way you get your accuracy, you don't have systematic errors

This is some Monty Python level maths comedy going on here!

I’ve thought about this part of the Christmas lectures at almost every government briefing. Fully worth watching (RI lectures, not necessarily Matt Hancock)

@randomdogdog

4 жыл бұрын

It's a classic science communication problem. (... and not science too) Normally you do it through the "in a study, doctors were presented with..." spiel. And yeah, solving people misunderstanding false positives/negitives is real research. The visual of the 17 presents at the end is stunning, and probably the best presentation I've seen yet.

@NoriMori1992

3 жыл бұрын

Who's Matt Hancock?

your delivery is so good in this. very charming and makes the point understandable. honestly look up to you on that

Of course a Parker Machine wouldn't be 100% accurate.

@gildedbear5355

4 жыл бұрын

but it gives it a go and that's what's important.

I remember going on a school trip to one of the 2013 lectures. I was 12, that's made me feel old.

@SlimThrull

4 жыл бұрын

When I was 12 Matt Parker had yet to be born.

That machine operates very smoothly, the craftsmanship is clearly top-notch. I'd love to see it taken apart.

That sorting machine reminds me of an old Soviet joke: The Americans build a potato peeling machine, and show it off at a conference where Gorbachev is present. It's TV sized box. They drop a potato. It ends up in the dispenser after a few seconds nicely peeled. Throw in two, a few seconds later they appear as well. They pour in a whole sack. Within 30 seconds all are perfectly peeled. Gorbachev goes home and gives the order that Soviet engineers have to develop their own peeling machine, so they can present it to the world too. They come back after barely a week. It's the size of a small car. They drop a potato, wait a minute and a much smaller edgy one slides out of the dispenser. They drop two, wait two minutes, and another two, small ones slide out. They pour in a whole sack. They wait ten minute. Then twenty. After half an hour a small paper slides out: "Sasha can't take it any more."

@nanigopalsaha2408

4 жыл бұрын

I don't get it. Is it the fact that they just cut out a lot of potato instead of peeling it? Or is it that Sasha is sitting inside and peeling them? What is the funny part?

@kane2742

4 жыл бұрын

@@nanigopalsaha2408 Sasha is inside peeling them, and doing a worse job at it than the American machine did.

@joshuacollins385

4 жыл бұрын

@@nanigopalsaha2408 The Parker machine didn't work very well and just has a person inside it, their joke was similar on those two points.

@ObjectsInMotion

4 жыл бұрын

I don't see how this is a joke, or funny at all.

@markdoldon8852

4 жыл бұрын

@@joshuacollins385 it worked perfectly, it demonstrated the problem. Its job wasnt to find phones.

80% of the time it works 100% of the time.

@danielyuan9862

4 жыл бұрын

100% of the time it works 80% of the time.

When your doctor administers you a test, they should tell you up front, "if you test positive, it really means you have an X% chance of having it. If you test negative, it really means you have a Y% chance," because X will often be actually pretty small. Then, if you test positive, your doctor should give you more tests to increase your certainty.

@HagenvonEitzen

4 жыл бұрын

But X and Y depend on a priori probabilities. A positive result of an 80% accurate test for a very rare desease performed test on a random person will almost always be a false positive. The same with a person from a subpopulation with a much higher probabilty of having the desease (e.g., someone with suitable symptons) will have a much lower false positive rate

6:12 - machine and presents 6:24 - empty stage

I read an article a few years ago about mammograms and their effectiveness... or maybe it was a video that I saw... regardless, it talked about how there were a distressingly high amount of false positives as well as a regrettable amount of false negatives. The math in the article/video brought into question the effectiveness of mammograms as a screening method.

I didn’t watch the original lectures even though I have a VPN, but I will watch them now that it’s legal. I even got a discount on my VPN subscription from your friend Destin. The only time I actually saw you in person you were with Destin both busy getting ready for Thinker Con whilst I was early looking about the museum. Destin turned aside from his business for about five seconds and said hi while you just kept going on with laser focus on whatever the important thing was that you were doing.

I really like the RI channel.. and gosh I love Hannah, and well.. you.. are a Parker in my hearth... (?

I died at “Xmas ray detector .80”

that poor child. imagine getting hyped at an 80% chance of getting a phone only to have it destroyed

you are really good at stage!

Elling (did I hear that name correctly) should've asked to rescan the wrapped gifts that came through from the first sort. That would've improved the selection odds.

I must have misheard the number, how accurate is the machine?

@U014B

4 жыл бұрын

Somewhere between 0% and 100%, if I remember correctly.

@Septimus_ii

3 жыл бұрын

He forgot to mention it, the machine is in fact 80% accurate

Fantastic demonstration!

The crushing disappointment as the child realised the phone was not coming...

Who did you get to work the inside the machine!? That's awesome.

The machine has produced a smaller pile of boxes with a higher % of phones. You could feed those back into the machine repeatedly until only one box remains, and it would have a much higher chance of being a phone than picking a box after just one pass. (Assuming, of course, that it's an actual machine and not just a box with a guy in it who switches out some of the boxes...)

@lumer2b

4 жыл бұрын

You can't know that. If the machine randomly select something as false / true with 80 % then yes, thus strategy would work. In real life usually tests fail because of some reason (it can't determine what's in the box; the medicine does not work for patients with a particular condition) so putting it again would wield the same result.

@MrRafalel

4 жыл бұрын

ARE YOU IMPLYING THE MACHINE ISN'T REAL?!?!?!!??! Heresy!

@John73John

4 жыл бұрын

@Liku Just take the gold-wrapped ones, tear off the paper, and put red on them again.

@JNCressey

4 жыл бұрын

You're assuming that, for any given box, the machine wouldn't just reproduce identical results each time.

Hey Matt, dunno if you'll see this: seems like a good follow up to this would be a discussion of Precision, Recall, Specificity, and any other relevant terms for this kind of thing.

Bit of a Parker Square of a machine.

Parker Mathematical Consultants "Where perfect is the enemy of good"

2:08 I only _just_ noticed the "X-Mas Ray" pun 😂

Bayes theorem, 3Blue1Brown - kzread.info/dash/bejne/eo57pdGPdpa_prA.html Crash course - kzread.info/dash/bejne/a4h4zMufoKrQXc4.html Veritasium - kzread.info/dash/bejne/hGVnpKZxoa7LhMo.html Zack Star - kzread.info/dash/bejne/amalldeSp5mzpbw.html

A bit of Fry and Parker!

I know I'm late to this, but I need to know what you told the red and yellow hats that was so funny.

Good job Matt and Hannah. Disease screening explained in 5 mins.

I've just seen you on 'World's Top 5' planes commenting on a spy plane. A repeat on Quest, but still found it interesting.

This video provides a nice visual explanation of the pitfalls of Covid-19 antibody testing with some of the current tests...

Loved the lectures. I doubt any will ever top the ones from the Polish(?) chemist gentleman for me though.

Hey I'm slightly early :) Love you Matt.

My idea of how your machine actually works is someone sitting inside there and exchanging certain boxes... Though I can't help from feeling bad when I think of that whole bag of boxes that landed on their head. Btw, were there actually any phones there?

@Septimus_ii

3 жыл бұрын

I don't think anyone was counting the number of dud presents going in and out, so the 'machine' just had to put most of the dud presents out and mix in 23 'phones'

@Maninawig

3 жыл бұрын

@@Septimus_ii do you think the maths loving Matt Parker, who loves accuracy in numbers, would go on stage making a mathematically inaccurate machine?

What would happen if one removes the gift wrap and put the box again in machiene ? Would the possibility of finding a phone increase or decrease or remain unaffected ?

Bloody hell Matt, it went from some kids excited to win an iPhone to kids worrying about their inevitable demise! That got dark quick!

Depending on how the machine works, maybe you can but the gold presents back into the machine with the bow removed and see if it comes back with or without a bow. Basically, re-test a box

We watched this in school on our last day before it closed

If you uploaded this yesterday the number of phones would match the date

It's like a Parker Square machine!

So, I followed the link to the full lectures, but it won't allow me to add them to a "watch later" list. Thanks KZread.

That poor volunteer will never again trust mathematicians bearing gifts

crossover of the century huh

This is a really good explanation of the prosecutor's fallacy

What if you fed the machine's output back into the input, and continuously refined your selection until only one present remained?

thats one parker square of a machine innit

I'm not accustomed to seeing Hannah with short hair.

Thank god you just filled 3 hours of my 24 hours allocated daily :D

Hey, I looked this sort of test, completely by coincidence, in one of my last stats classes

Will, you also post the thing you did with vaccinations at the Christmas lectures?

I thought they are gonna put the possible phones again to the machine to the point there is only one left. That should be pretty good strategy, right?

@nijnij3988

4 жыл бұрын

I was thinking the same thing! Because your chances of finding a phone did increase with the machine, because before the machine 5% of all the presents were phones, and after the machine 17% of the rewrapped presents were phones. If you ran those through the machine a second time, wouldn't that make your chances almost 50/50? (3.2 phones and 3.8 rubbish presents)

@digitig

4 жыл бұрын

Only if the errors are independent on each pass. If there is a reason it thinks something is a phone when it isn't, it's likely to think the same thing on the next pass too.

@aok76_

4 жыл бұрын

But then what about the ones that were mistakingly thought to not be phones but actually were. After all, it's 80% accurate. I think one labor-intensive way would be to retest all the boxes and give a score to those that pass. With each iteration, the score of the phones would rise since they are more likely to be chosen as correct every time.

@RodelIturalde

4 жыл бұрын

@@aok76_ indeed, if tests are independent. Though, since it is 1 machine that identifies something as 'phone-worthy'. It is likely the same machine will label same something 'phone-worthy' again. Machines that identify often tries to check a few things. If these few things match up with a positive marker, it gets a positive marker, phone in this case. So when Re-testing same object with same machine, it is likely it will pick up on same identificatiors and label it as positive again. Regardless of wether it is a phone or just a look-a-like.

@pyglik2296

4 жыл бұрын

@@nijnij3988 Exactly! If you assume independent testing and calculate it through the mean value you get 17.4%, 45.7%, 77.1%, 93.1%, 98.2%, 99.5% and finally after seventh iteration you're left with average 1.05 phone and a chance of nearly 99.9%!

Ah it's a real parker square of a machine.

I would have put the 'phones' in the machine again. Just to mess with Matt.

I liked those gold Parker Cubes it spat out.

It feels like you should have written the accuracy rating on the side, or had a big sign. You know, just to make sure everyone knows how accurate we're talking about.

A real Parker Square of a machine

That's a real Parker Square™ of a machine

80% of the time, it works *every* time

Classic Parker square

Well i already seen the full lecture but still fun

So it's a Parker square of a Present scanning machine!

Ever since I started paying attention to “testing” I have been remembering this part of the lecture and wondering how accurate tests are. I haven’t heard anything about accuracy but I want to.

@SimonBuchanNz

4 жыл бұрын

The one number I've heard is a 1/3 false negative! Even the Parker machine does better! Though really, for a pandemic I would guess you're better off having a low false positive, if you're trading the two off (which you often are when designing the tests), so it might not be *as* bad as it sounds...

@extrastuff9463

4 жыл бұрын

I haven't kept track of things much, but I've come a few policy mentions in the news and online here and there that bring up two tests. What test is being used varies widely, but I can only guess they are using a quick test that is reliable at finding true positives and potentially a decent number of false positives. If that first test comes back positive some different or more elaborate test is used which has a high specificity (true negative rate) as well. But there are quite a few different methods and implementations of tests floating around, in the end you don't have much of a choice anyway and whatever your local hospital or public health institute uses is almost certainly the test that you'll get. I do hope that people interpreting the tests have a good idea of how statistics work and what the relevant values are for their tests. Especially false negatives seem dangerous in a virus context, a false positive isn't fun either but a second test/close look at the symptoms should remedy that concern. That's actually one of the reasons why I'm a bit worried about those tests one can order on their own that seem to pop up in various countries, I'm very sceptical about the general population having enough understanding of statistics. Even if the test results actually include an explanation about it they are likely to ignore it and false negatives are dangerous.

If it's not 100% deterministic, and therefore you could rescan item, how many run throughs would it take to get to some percentage, e.g. 95%?