Chesterton Tribune



Difference between all-test and unique-individual test-positivity rate

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The Porter County Health Department (PCHD) is currently reporting, on its daily COVID-19 dashboard, two different kinds of test-positivity rate, one of which produces a typically lower rate, the other a higher rate.

As of Wednesday, PCHD was reporting, as of Oct. 13, an “all-tests” seven-day rolling-average positivity rate of 8.1 percent. It was also reporting, again as of Oct. 13, a “unique-persons” seven-day rolling-average positivity rate of 11.6 percent.

The difference between the two kinds of test-positivity rate is significant: nearly three and a half points.

The “all-tests” rate measures the percentage of positive test results out of the total number of tests given in a specific population. Many people--healthcare workers, for example--are tested multiple times, and every one of those tests counts toward the total number of all tests given.

The “unique-individual” rate, on the other hand, measures the percentage of individuals testing positive out of the total number of people tested in a specific population. Those people are only counted once, however. That is, an individual might test positive for COVID-19 after testing negative 10 times, but only the positive result, not the 10 negative ones, count toward the positivity rate.

The “all-tests” positivity rate is an excellent metric for determining how widespread and accessible COVID-19 testing is in a community, but because the overall pool of test results includes so many negatives it tends to mask somewhat the spread of infection in a community.

The “unique-individual,” in contrast, is probably a better snapshot of the extensiveness of infection in a specific population.

As PCHD notes on its dashboard, with respect to both the “all-tests” and the “unique-individual” measure, the “positivity rate may change as additional test results, both positive and negative, are submitted.”


Posted 10/22/2020




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