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Contrary to Johns Hopkins report, Idaho reports 9% coronavirus positivity rate, not 23%

Johns Hopkins University reports Idaho has the second-highest positivity rate in the nation, but a state epidemiologist explains the number used is inflated.

BOISE, Idaho — New data from Johns Hopkins University shows that Idaho has the second-highest COVID-19 positivity rate out of all 50 states at just over 23%.

If you look at Idaho state data though, the positivity rate is listed at only 9%. That number is still relatively high, but a major disparity from the rate reported by Johns Hopkins.

Idaho deputy state epidemiologist Dr. Kathryn Turner explains the difference in calculations.

"Johns Hopkins uses data that's available publicly because that's the data they have," Turner said. "They take the total number of cases we have and that's their numerator, and they take the total amount of lab tests that have been done and that's their denominator and it's a pretty straight-forward calculation."

But according to Turner, the state’s calculation is simply more precise.

"We actually go through and really clean the data," she said. "We only include lab reports from laboratories that send us both positive results and negative results - so that the numerator and the denominator are pulling from the same basket. 

"So if we were to include positive results from labs that don't send us all of their negative tests, it would inflate the numerator, and then of course our percent positivity would be higher," Turner added.

Essentially, it’s two different math equations with an answer that is labeled the same.

"Exactly, it's different data," Turner said.

The Johns Hopkins math isn't wrong, Turner explained, it's simply a result without the same data set.

"They are using the data they have access to publicly," she said. "It's a little more complicated than just throwing two numbers together and dividing. There is nothing wrong with their calculation based on the data they have access to, it's just that the way they do it, it ends up inflating our data."

So with a 14% disparity between 9% and 23%, where does the actual positivity stand?

"It's closer to 9.3%," Turner said. "We spend basically a whole week, that's why we only do the data once a week, we spend a week of staff time cleaning every single record to make sure that those negative results are Idaho residents."

Being precise and making sure all data in the equation is correct is crucial.

"There is policy decisions being made off of this data and we can't just guess and we can't just throw two numbers out there," she said. "So, we do everything we can to make sure it's as precise as possible and that's why we take the time to do it."

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