One of the key metrics used to measure the spread of COVID-19 is the spot positivity rate.

When Gov. Phil Murphy announces the spot positivity for New Jersey during his thrice-weekly news conferences, the number doesn't reflect the reality of test conducted the previous day.

According to Dr. Ed Lifshitz, the director of communicable disease services for the state Department of Health, the spot positivity refers to the percentage of positive coronavirus tests from the entire number of individuals tested for the virus on a particular day.

Lifshitz said this number helps to give health officials, state leaders and members of the public an idea of how prevalent COVID-19 is in New Jersey, but it’s really a snapshot of what the testing situation was four days ago.

On Friday, Murphy noted that the spot positivity rate on Aug. 10 was 1.63%.

Why look at four days ago?

“We choose four days because in that time we’ll have gotten most — but not all — of the results back," he said. "We will have gotten enough of the results back that we’ll have a good idea of the percentage of who would have been positive.”

This means that snapshot in time may not be completely accurate because it will not include any test results that had not been completed within the previous four-day period.

“We can’t be sure we’ve gotten everybody who was positive and everybody who was negative,” said Lifshitz. “But we’ve gotten enough of them back that we’re comfortable in saying that we have a good idea of what the overall percentage of them that are positive would be.”

He noted last month, when COVID-19 was spiking in other states and labs in New Jersey were taking a week or longer to get test results back, the spot positivity may not have been as inclusive, and perhaps not as accurate, because up to 40% of test results looking four days back were still pending.

He said that when devising a formula for measuring the spot positivity rate, the four-day test result look-back was selected because “you want to get as close as you possibly can to that date so the numbers still mean what’s going on now, but far enough away that you get enough of the results that the numbers are meaningful.”

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