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.”