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On March 22, 2021, we detected that Florida was no longer reporting "testing encounters" (tests_encounters_total), which served as the denominator in our "testing positivity" calculation. As of March 26, 2021, we began using “total viral tests” (tests_viral_total) as the new denominator and applied that calculation to all previous data.
Our data source for test data, the Covid Tracking Project, changed its definition of the contents of its totalTestResults field for Florida. We use this data for our positivity calculation denominator. This will will likely reduce the state's positivity percentage because people are deduplicated people less frequently, resulting in a bigger denominator.
Previously, the totalTestResults field included Positive Cases + Negative Test Results. Now, the field includes Test Encounters, which are defined as the total number of people tested per day via PCR testing. The count for this metric is incremented up by one for each day on which an individual person is tested, no matter how many specimens are collected from that person on that day. If an individual person is tested twice a day on three different days, this count will increment up by three.
Recently, we have fielded inquiries about inconsistencies between JHU's and Florida's positivity calculation. There are several potential factors. Per Florida’s data definitions, there is an inconsistency in how the tests are deduplicated. For total cumulative negatives, all tests are deduplicated so people only show up once as either positive or negative. For weekly negatives, which is used in Florida’s weekly positivity calculation, tests are deduplicated for people tested that same week, allowing people to duplicate from week to week.
Also, Florida calculates positivity only using Florida Residents, while we use the total number of people tested in Florida. Ultimately, it is difficult to pinpoint the discrepancy because Florida does not make all of the variables that feeds into these calculations available, nor does the data definition provide the calculation for its Positivity Rate of New Cases by Week.