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