This metric ignores the number of people testing positive. Instead, it focuses on the odds of a positive test result.

In other words, out of everybody who has been tested, how many results were positive?

A lower rate is better. It’s a figure the state said it’s watching to help decide whether to reopen the state more or — if the rate begins a “sustained” rise — to tighten restrictions.

For now, rates for all of the state’s regions have been dropping, a good sign.

As with case numbers, though, the availability of testing can complicate the picture.

At the beginning of the pandemic, positivity rates were high, but only because testing was being conducted mainly on people who were likely to have the virus.

“Imagine that there’s so many people who are infected, and you’re testing so little, that every time you pull somebody off the street, you find that they’re positive. It means you have very little idea on what is happening to the epidemic,” Goldenfeld said.

Now, the rates are far lower. In fact, the positivity rate is so low in central Illinois that Goldenfeld said it appears the state is “crushing” the virus there.

Still, the testing issue makes it difficult to use this metric to measure the infection’s trajectory over time. While it’s good that testing has been opened up to more people, the change means that today’s rates can’t be compared with prior ones to assess whether the pandemic is getting better or worse.

In a perfect world, researchers say, the state could randomly test people periodically as a way to quickly and accurately estimate how many people are infected. Think of it like polling, but for a virus.

But that would be expensive, and testing supplies still aren’t that plentiful. So researchers from the University of Chicago and Northwestern University submitted a draft paper last month to the state suggesting that a couple of key groups be regularly tested: women giving birth, and anyone with COVID-19 symptoms seeking outpatient care.

Some of this is already occurring. Edward-Elmhurst has been testing women giving birth at its two hospitals since April 13. But it’s unclear how many other hospitals are doing the same.

University of Chicago researcher Sarah Cobey, who’s working with Northwestern’s Jaline Gerardin on last month’s proposal, said they’re waiting to hear back from state health officials and the governor’s office.