Jump to content

Menu

The predictive value of COVID positivity


Recommended Posts

5 minutes ago, Happymomof1 said:

Ok, the rate I am seeing for SD is 23.2 percent.  So divide 895,000 by 3 and you get 298,333.  Now you  multiply that by 23.2 and you get 69,213.   Now that should be in about a month if I understand it.  I was using this to find the percentage:  https://covidactnow.org/?s=1304314

I use John Hopkins. Here is what they show for SD - well over 50%.

And here is a summary of positivity rates by state, based on JH data.

Not looking to quibble over it, just backing up my stats above.

Edited by Happy2BaMom
  • Like 2
Link to post
Share on other sites
  • Replies 417
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Popular Posts

Wow. Different here. I  just came back from picking up Hobbes (Scotland to England and back). Empty motorways and when I  picked up coffee in the morning, I  was the only customer.

Hmm.  That looks a lot more like the death curve.  In fact, if we take that one and advance the dates two weeks and multiply everything by two (the red line) we get a pretty good predictor of what dea

Polio is asymptomatic in 99% of people...

Posted Images

On 10/18/2020 at 6:38 AM, Pen said:

 

MAnd while I don’t really think we could have been like Sweden, it is looking pretty good at the moment

Worldometer daily deaths Sweden:

FC120B97-9DDB-438C-8CAA-F6E47BEC6236.png

Slowly ticking back up now unfortunately although they are introducing some measures to slow it down.

 

And here is the cases.  Deaths lag is 4-6 weeks of more so this could be quite scary in weeks to come.  Obviously the earlier spike is probably underrepresented due to lack of testing.

 

 

 

 

 

 

 

C28CAB25-71EF-4789-9744-8EE8708007BB.jpeg

C9A5A7A1-4B20-4D38-A067-D38710DF1115.jpeg

Edited by Ausmumof3
Tidying up my screen shots
Link to post
Share on other sites
12 minutes ago, Not_a_Number said:

No, population in millions. So for SD that'd be 0.9 or so, not what you did. And you leave the positivity number as how many percent it was. So then you get 

0.9/3*50 =15, 

which seems pretty plausible, I gotta say. 

Is that deaths per day?

  • Thanks 1
Link to post
Share on other sites
8 minutes ago, Ausmumof3 said:

And here is the cases.  Deaths lag is 4-6 weeks of more so this could be quite scary in weeks to come.  Obviously the earlier spike is probably underrepresented due to lack of testing.

Not generically. I have not seen countries for which this is true, whatever people say. 

Link to post
Share on other sites
3 minutes ago, Not_a_Number said:

Not generically. I have not seen countries for which this is true, whatever people say. 

Hmm I wonder how that works?  US previous case peak was roughly July 24 and deaths about August 4 so 10 days to 2 weeks.  Maybe more people start seeking testing when people get hospitalised?  I’m just curious I mean we know it takes time to die so if cases and deaths peaks at the same time there must be some human behaviour factor driving that.

Link to post
Share on other sites
1 minute ago, Ausmumof3 said:

Hmm I wonder how that works?  US previous case peak was roughly July 24 and deaths about August 4 so 10 days to 2 weeks.  Maybe more people start seeking testing when people get hospitalised?  I’m just curious I mean we know it takes time to die so if cases and deaths peaks at the same time there must be some human behaviour factor driving that.

From what we've seen on this thread, 2 weeks is about right. I think @Danae ran correlations between positivity and deaths and that was the best she could do. I think it takes a while for people to get tested, and then for tests to be reported. So the case to death lag for almost every country I've monitored is 2-3 weeks. 

Edited by Not_a_Number
Link to post
Share on other sites
1 minute ago, Not_a_Number said:

From what we've seen on this thread, 2 weeks is about right. I think @Danae ran correlations between positivity and deaths and that was the best she could do. I think it takes a while for people to get tested, and then for tests to be reported. So the case to death lag for almost every country I've monitored is 2-3 weeks. 

Interestingly with Australia there is a month between case peak and deaths peak.  So maybe when there’s adequate testing and a solid push for people to seek testing and a population that seeks testing the data is more in line with that.

  • Like 1
Link to post
Share on other sites
3 minutes ago, Ausmumof3 said:

Interestingly with Australia there is a month between case peak and deaths peak.  So maybe when there’s adequate testing and a solid push for people to seek testing and a population that seeks testing the data is more in line with that.

Not seeing that in the Worldometer graph. Seeing 2-3 weeks: 

https://www.worldometers.info/coronavirus/country/australia/

ETA: ignore the giant peaks, though, measure to the middle. Giant peaks are data someone is dumping late. 

Edited by Not_a_Number
Link to post
Share on other sites
2 minutes ago, Not_a_Number said:

Not seeing that in the Worldometer graph. Seeing 2-3 weeks: 

https://www.worldometers.info/coronavirus/country/australia/

ETA: ignore the giant peaks, though, measure to the middle. Giant peaks are data someone is dumping late. 

Using 7 day average I’m seeing August 5 and September 4.  We did get daily reports on deaths and cases aside from one data dump from aged care one day so I think the data is reasonably accurate.

Link to post
Share on other sites
2 minutes ago, Ausmumof3 said:

Using 7 day average I’m seeing August 5 and September 4.  We did get daily reports on deaths and cases aside from one data dump from aged care one day so I think the data is reasonably accurate.

That's what I'm saying: focus on the actual shape of the graph in gray, ignore the single day data dump. That data dump is NOT from that day.  

Where's the middle? 

Edited by Not_a_Number
Link to post
Share on other sites
1 minute ago, Not_a_Number said:

Yeah, or I might actually say August 23 -- I'd guess the low numbers there are an artifact. But either way, you can see it's not 4-6 weeks. 

But I’m curious to know why?  We know that positive test to illness to death takes time.  Is it an artefact of lack of testing or something else?

Link to post
Share on other sites
Just now, Ausmumof3 said:

But I’m curious to know why?  We know that positive test to illness to death takes time.  Is it an artefact of lack of testing or something else?

I actually don't know how long death takes on average, do you? Recovery takes a while, but death might on average not take that long. (Or, on median. You don't really want to count outliers here.) 

Link to post
Share on other sites
14 minutes ago, Not_a_Number said:

I actually don't know how long death takes on average, do you? Recovery takes a while, but death might on average not take that long. (Or, on median. You don't really want to count outliers here.) 

21 days is what I’m seeing most but most of the data is from March April.  That may have changed  with better treatment protocols.

Link to post
Share on other sites
Just now, Ausmumof3 said:

21 days is what I’m seeing most but most of the data is from March April.  That may have changed  with better treatment protocols.

Matches Australia's data well, though 😉 . 

Most people don't die, so I don't know how much treatment is helping the people who will. The death rate is largely in very old and frail people. Many more people get very sick than die 😕 

  • Like 1
Link to post
Share on other sites
11 minutes ago, Ausmumof3 said:

For what it’s worth the death lag of three weeks quoted is from here 

so I have no idea where he’s getting that from but I didn’t make it up 🙂 just didn’t check it out.

 

Yeah, he's kind of a publicity hound. Not a fan. 

ETA: he's probably right about 3-4 weeks, though, lol. I shouldn't let the antipathy prejudice me!! 

ETAA: actually, if he's saying that's what it looks like on the graphs, he's wrong. 

Edited by Not_a_Number
Link to post
Share on other sites
29 minutes ago, Not_a_Number said:

Yeah, he's kind of a publicity hound. Not a fan. 

ETA: he's probably right about 3-4 weeks, though, lol. I shouldn't let the antipathy prejudice me!! 

ETAA: actually, if he's saying that's what it looks like on the graphs, he's wrong. 

Lol yeah well he’s not the only one just the easiest one to find.  Have seen it from multiple sources.  But you’re right from looking at the peaks it’s more like three weeks with good testing and two weeks with less.  I’m also guessing the lead time is probably less accurate during the peaks because lack of testing means people get tested later in the infection.  

  • Like 1
Link to post
Share on other sites
1 minute ago, Ausmumof3 said:

Lol yeah well he’s not the only one just the easiest one to find.  Have seen it from multiple sources.  But you’re right from looking at the peaks it’s more like three weeks with good testing and two weeks with less.  I’m also guessing the lead time is probably less accurate during the peaks because lack of testing means people get tested later in the infection.  

Yeah, that was my impression: lag was shorter for peaks than troughs.

Edited by Not_a_Number
  • Like 1
Link to post
Share on other sites
On 10/15/2020 at 3:51 PM, Not_a_Number said:

How can you ever know? I've followed it long enough that it's a fairly robust pattern. If I did run statistical tests on it, it would be far too strong a correlation to be random. But I'm not going to bother, because I'm not publishing a paper -- I'm just sharing what I've observed. 

As for the data, it's not awesome, I agree. But then that's how it goes in a pandemic -- the data isn't awesome. This thread is just for people who are interested in what is the most predictive, and so far, it seems to be positivity and not other things. I've personally found that way of thinking about it helpful for my own decisions. 

But I don't understand how your positivity rate for any given date could possibly correlate with deaths for 2 weeks later.  If the majority of tests are done when people begin symptoms, the ones that die are not going to die two weeks later.  I believe that Dr Campbell said that for most people, deaths happened between 4-6 weeks after they tested positive?  

I must be missing something about how you are thinking about this.   

Link to post
Share on other sites
2 hours ago, Halftime Hope said:

But I don't understand how your positivity rate for any given date could possibly correlate with deaths for 2 weeks later.  If the majority of tests are done when people begin symptoms, the ones that die are not going to die two weeks later.  I believe that Dr Campbell said that for most people, deaths happened between 4-6 weeks after they tested positive?  

I must be missing something about how you are thinking about this.   

You are assuming that cases are reported immediately, which I doubt -- I'm sure there's a backlog. We also couldn't find reliable data about how long it takes people to die. 

Either way, the way I'm thinking about this is that I looked at a lot of countries, and every single country had a lag of 2-3 weeks from cases to deaths. For the US, that lag is not from case numbers but from positivity. You can see the graphs yourself -- they are somewhere on this thread. If you need a link, I'll link them up. 

Link to post
Share on other sites
On 11/10/2020 at 12:06 AM, Happy2BaMom said:

I use John Hopkins. Here is what they show for SD - well over 50%.

And here is a summary of positivity rates by state, based on JH data.

Not looking to quibble over it, just backing up my stats above.

Wow.  Those positivity rates are high. Scotland is at 9 percent and we are only allowed out for essential purposes and aren't allowed to enter other homes.

  • Like 3
  • Thanks 1
Link to post
Share on other sites

Positivity is above 9%. That’s double the bottom of the last trough. Expect to hit 2000 deaths per day on average in December, although I would guess not November. Current prediction for end of November is about 1500 deaths/day on average.

Edited by Not_a_Number
  • Sad 4
Link to post
Share on other sites

@Danae, are you still keeping track? Has the scaling constant gone up? The one for the US overall definitely has, which makes a lot of sense.

@Corraleno, these correlations are why I tend to think we aren’t on track for 2000 deaths/day on average all that soon, although I DO assume we’ll hit that within December unless something changes.

  • Like 1
Link to post
Share on other sites
3 minutes ago, dmmetler said:

My area seems to have stabilized at 10% BEFORE people start gathering for holidays and stores start extended hours for holiday shopping. I'm very worried about what the next two months will hold. 

Yeah, I’m seeing some hints of US-wide stabilization... but you’re right that the holidays are next week 😮. I didn’t even think of that.

  • Sad 1
Link to post
Share on other sites

Here's @Danae's rough estimate — take the population of the region in millions and divide by three. Multiply that by the positivity rate today to get the death count two weeks from today.

Trying this for NY: 2 weeks ago, the positivity rate was something like 1.7%. This prediction gives 

1.7*20/3 = 1.7*6 = 10.2 

Definitely WAY too low. But then one does rather expect that for the states which are doing plenty of asymptomatic and precautionary testing. 

Link to post
Share on other sites
Just now, Not_a_Number said:

Where are we talking about? I've lost track. 

That was the prediction for South Dakota.

According to the chart that was posted upthread, US-wide positivity was only 7.6% 2 weeks ago (11/6). That gives a prediction for today of around 840. The average yesterday was already up to 1398, so that's way off.

Link to post
Share on other sites
19 minutes ago, Corraleno said:

That was the prediction for South Dakota.

According to the chart that was posted upthread, US-wide positivity was only 7.6% 2 weeks ago (11/6). That gives a prediction for today of around 840. The average yesterday was already up to 1398, so that's way off.

Yeah, that's not surprising -- I've been noticing the scaling factor ticking up. It's because we're testing many more asymptomatic people is my guess... the college and workplace testing. 

But that shifts pretty slowly, so the best way to predict what happens in 2 weeks is usually to just take the CURRENT scaling factor and use it. According to that, we'll be at about 

10/7.6*1398 = 1839

deaths in 2 weeks on average. So, not what I call good news, lol... but also less than you predict if you just use case numbers. 

We'll see if I'm right, of course. So far, it's tracked well, though. 

Link to post
Share on other sites

Of course, South Dakota is totally not testing asymptomatic people, lol! Their numbers are a total disaster. 

I don't know what to do with their data, frankly. It's very spiky. Frankly, given the quality of their data collection, being anywhere near in the ball park is good enough for me. 

Edited by Not_a_Number
  • Like 1
Link to post
Share on other sites
7 hours ago, Not_a_Number said:

@Danae, are you still keeping track? Has the scaling constant gone up? The one for the US overall definitely has, which makes a lot of sense.

@Corraleno, these correlations are why I tend to think we aren’t on track for 2000 deaths/day on average all that soon, although I DO assume we’ll hit that within December unless something changes.

It's gone way up. 

  • Like 1
Link to post
Share on other sites
4 minutes ago, Not_a_Number said:

How much? Theories why?

I said a few weeks ago that the multiplier seems to go up when positivity is above 5.  Which makes sense with what epidemiologists have said that at 5% you know you're doing enough testing.  Above five means you aren't testing enough, which means a higher % of cases are being missed.

 

  • Like 1
Link to post
Share on other sites
3 minutes ago, Danae said:

I said a few weeks ago that the multiplier seems to go up when positivity is above 5.  Which makes sense with what epidemiologists have said that at 5% you know you're doing enough testing.  Above five means you aren't testing enough, which means a higher % of cases are being missed.

 

Except that if we think of it as sampling, sampling MORE sick people ought to get the scaling factor down, not up... (Sampling asymptomatic people drives down positivity and all.)

I was keeping track of the factor calculated every week before, but sadly it got lost in the purge. It’s possible that the lag changes when there’s an outbreak, too? So we need to look fewer than 2 weeks back.

Link to post
Share on other sites

I think that when the numbers are this huge and cases cover the entire US, cases x CFR is not a bad predictor. And obviously the CFR changes in line with positivity, so case-based predictions include positivity in that sense. 

Predictions for the first two weeks of November based on cases three weeks earlier x a CFR of 1.7% were absolutely spot on, and for this week the prediction was low — 1380, when we hit 1398 yesterday and will be well into the 1400s by the end of today. So that suggests that the CFR is creeping up, closer to 1.8, which implies we're going to be well over 2000 in 2 weeks (12/4). I think the numbers are going to be all over the place next week, with what's essentially a 4-day holiday weekend , but I think the numbers the week after are going to be really grim. 😕

Link to post
Share on other sites
11 minutes ago, Corraleno said:

I think that when the numbers are this huge and cases cover the entire US, cases x CFR is not a bad predictor. And obviously the CFR changes in line with positivity, so case-based predictions include positivity in that sense. 

Predictions for the first two weeks of November based on cases three weeks earlier x a CFR of 1.7% were absolutely spot on, and for this week the prediction was low — 1380, when we hit 1398 yesterday and will be well into the 1400s by the end of today. So that suggests that the CFR is creeping up, closer to 1.8, which implies we're going to be well over 2000 in 2 weeks (12/4). I think the numbers are going to be all over the place next week, with what's essentially a 4-day holiday weekend , but I think the numbers the week after are going to be really grim. 😕

Do you want to see what my predictions would have been? I can test what I’d have done and see how well it’s performing. The problem is that the CFR is highly dependent on the fraction of caught cases.

Link to post
Share on other sites

I wonder if the spike has anything to do with Halloween. We are in CA and my county has been in the purple (most restrictive) tier the entire time. We haven’t loosened any guidelines here locally and yet despite doing what we have been doing all along, we are still experiencing a spike. It’s very frustrating.

  • Like 1
Link to post
Share on other sites
8 minutes ago, Not_a_Number said:

Do you want to see what my predictions would have been? I can test what I’d have done and see how well it’s performing. The problem is that the CFR is highly dependent on the fraction of caught cases.

Yes, that would be useful.

CFR of 1.7% x cases 3 weeks ago = 1380
CFR of 1.75% x cases 3 weeks ago = 1420 
CFR of 1.8% x cases 3 weeks ago = 1459

Edited by Corraleno
  • Like 1
Link to post
Share on other sites

I've been playing around with numbers, and it looks like maybe what changes as the positivity goes higher is both the scaling factor and the lag.  Which is, unfortunately, much harder to model in a spreadsheet.  

 

Edit:  but is consistent with the idea that higher positivity means things are out of control and testing isn't keeping up.  

Edited by Danae
  • Like 1
Link to post
Share on other sites
9 hours ago, Not_a_Number said:

@Danae, are you still keeping track? Has the scaling constant gone up? The one for the US overall definitely has, which makes a lot of sense.

@Corraleno, these correlations are why I tend to think we aren’t on track for 2000 deaths/day on average all that soon, although I DO assume we’ll hit that within December unless something changes.

Was 1800 per day two days ago and over 2000 yesterday.  Rolling average is at high 1300s which I assume is what you’re talking about.  Coming into the weekend you should have a couple of lower days potentially as several states don’t report deaths over the weekend.

  • Like 1
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...