About those "Corona virus positivity rate" percentages ...

[kenny]

Please correct me if my assumption is wrong, but ...

When I read, "Area X has a 40% positivity rate." it does not mean 40% of the people in Area X have Covid.
Rather, it means 40% of those tested have it.

Obviously 100% of people in any area have not been tested.
If, say, 10% of people in area X got tested and 40% of those tests were positive it means Area X has a 4% positivity rate - and a 96% unknown rate.

Right?

Next are those numbers cumulative since the beginning back in March 2019, or are they for testing only yesterday, the last week, the last month?
Anyone know?

 

[FL_runner]

Yes it’s the percentage positive of the tests that were run. It adds quite a bit of nuance when interpreting rates. A low positivity rate suggests that testing is ample, for example- a highly positivity rate may suggest that there are limited tests and they are being saved for very sick folks.

In your example above if 10% of people are tested and there is 4% positivity it’s 4% of the 10% tested so it’s not 4% of the overall population it should be 0.4% of the population (ugh math). 90% of the population is unknown because they were not tested.

it’s helpful to update the positivity rates frequently because you can infer whether rates will heat up/improve. In my area it’s updated off of current rates, we usually look at one week at a time.

 

[KristinTech]

Please correct me if my assumption is wrong, but ...

When I read, "Area X has a 40% positivity rate." it does not mean 40% of the people in Area X have Covid.
Rather, it means 40% of those tested have it.

Obviously 100% of people in any area have not been tested.
If, say, 10% of people in area X got tested and 40% of those tests were positive it means Area X has a 4% positivity rate - and a 96% unknown rate.

Right?

Next are those numbers cumulative since the beginning back in March 2019, or are they for testing only yesterday, the last week, the last month?
Anyone know?

My area posts a seven day rolling positivity rate. So it helps to put things into perspective as time goes on. I live in a college town, so things spiked in August, then settled down for awhile until early November.

 

[qubitasaurus]

Yes the above is spot on.

But the reason people track positivity rates is because when you have a very high positivity rate it suggests your testing system is really overwhelmed with an insufficient suply of tests. And probably there is a lot of people who have covid but simply werent tested at all.

In these places people will say X confirmed cases are being reported. But probably there are far more than X (they will use the test positivity rate to project how many more than X there are.). Itll become important if you really want to do any statistical analysis or maybe even logistics style planning. As there is good reason to believe that youve only got partial data from the tests.

 

[kenny]

... are those numbers cumulative since the beginning back in March 2019, or are they for testing only yesterday, the last week, the last month?
Anyone know?

Opps
Sowwy
Duh
My bad!

I meant March 2020.

 

[Karl_K]

There are 2 numbers they look at 24 hour average and the 7 day rolling average positivity rate .
They are looking at the trends a single rising 24 hour may be a fluke a rising 7 day rolling average is a bad sign.

The other and more important number is open icu beds available.
My area is at 79% icu usage with some hospitals at 90%+

 

[LilAlex]

Positivity rates reflect prevalence and the local threshold for testing. At the beginning of the pandemic, positivity rates were high because no one got tested unless it was pretty clear they had COVID. In the well-run areas of the globe that do good surveillance, positivity rates have been quite low.

The actual number of true positives can underestimate prevalence in the event of overly-restrictive testing (or no testing at all). Positivity rates (#positive/#tested) can provide an additional metric but I don't like it; it is also very susceptible to bias and to test availability.