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Here’s what you need to know about positive test rates, the key coronavirus stat that people are talking about

A test was conducted in late June at a North End walk-up site. An increase in the percentage of tests that are positive has people worried.Craig F. Walker/Globe Staff

The percentage of people in Massachusetts who are found to be infected when they are tested for coronavirus is back in the news nowadays. Here’s what you need to know:

While it is one of a group of key metrics the state says it is monitoring, its move upward has received repeated mentions from Governor Charlie Baker and other officials in recent days.

At a Tuesday news conference, Baker said that “over the past several days we’ve seen a modest uptick in the percentage of new positive cases and we continue to closely monitor and analyze the data to determine the factors that are driving that.”


He also warned, “We’ve seen some slippage in a few places, and I really hope we don’t see that going forward because it is going to affect our positive test rate, and it is going to affect the way we move forward – has to,” he said.

The percentage of positive tests is reported every day by the state on Page 2 of its coronavirus dashboard. A chart displays the “7 Day Weighted Average of Positive Molecular Test Rate.”

As of Tuesday, the chart showed the rate reaching as low as 1.7 percent in mid-July and edging upward to 2.2 percent for the five days ending Monday.

Experts say the percentage of positive tests is a crucial metric that, if it ticks upward, is a sign of increasing community spread of the virus.

“That is kind of the predictor that if you don’t do something — namely, do something different — if you’re opening up at a certain pace, slow down, maybe even backtrack a little,” Dr. Anthony S. Fauci, the nation’s top infectious disease expert, told The Washington Post last week.

What exactly does the state’s number reflect?


The number shown on the dashboard is a daily recalculation of the percentage of tests that were found positive in testing for the past week. The total number of positive tests for seven days is divided by the total number of tests administered.

It’s essentially the state’s testing “batting average” for the most recent seven days.

As of Tuesday, it indicated that 2.2 out of every 100 people the state tested for the coronavirus over the past seven days were found to be infected.

It is based on tests as of the date they were given, not the date the results were reported. The dashboard displays 30 days of the critical metric. The percentages reported on the chart can change from day to day, as more test results are received and assigned back to the days the tests were administered.

At least one well-known website that offers calculations of the positive test percentage appears to use a slightly different methodology, relying on total testing and positive testing data as of the day tests and results were announced, rather than the day the tests were actually conducted. That method has produced a higher percentage in recent days (up to 3.1 percent).

But Nicholas Reich, a professor at the University of Massachusetts who specializes in epidemiology and biostatistics, said he preferred the state’s method.

“My feeling is that it’s better, if you have the data, to do it by the date the test was done,” he said. “It gives you a sense of what was going on on that particular day that the test was conducted.”


Martin finucane can be reached at martin.finucane@globe.com. John Hilliard can be reached at john.hilliard@globe.com.