Plus, the number that tells whether a city, county, or state is safe to reopen.

As the rate of new coronavirus cases and deaths in the US slows down, many states and cities are encouraging businesses to open again, easing the lockdowns that have been in place since March. But some of those places might be doing so prematurely, according to an important metric that public health experts say signals when a place has tested enough, and in the right way: the test-positivity rate.

The test-positivity rate is the percentage of Covid-19 tests that come back positive relative to the total number of tests performed. Public health experts say it can be understood as a measure of how well a place has tested its population and to what degree it has identified a significant number of asymptomatic cases. A place with a