Florida’s peak week for the number of tests and new cases ended on July 17. Even though Positivity on that week was a staggering 19.0%, which screams-out for additional testing, the state cut back on test counts the next week, and for the next five consecutive weeks. Between July 17 and August 28, weekly testing levels dropped by 60%.
It wasn’t until the week ending September 5, in the face of increased positivity, that Florida’s testing counts increased, but modestly, still 53% below the July 17 peak.
Trump has been widely ridiculed in the press for saying that “more testing leads to more cases”. In the common sense meaning of these words that may seem like a stupid statement, but add one word and it becomes completely true: “more testing leads to more reported cases”.
In this post we’re going to estimate the impact of Florida’s decision to cut testing counts in reducing the Confirmed Case count. Here we calculated an alternative scenario where testing remained at the July 17 levels. As you can see in the chart, Floridas actual testing peaked at 459,151, and dropped steadily to 180,465 for the week ending 28 August, and then increased slightly to 216,130.
If Florida had maintained peak testing levels, how many additional cases does this add? That depends on the Positivity Level, i.e. the percent of people tested who test positive. We calculated using the CDC recommended methodology (see this post for a deep dive on how Florida plays games with the Positivity Level it reports on its dashboard).
To estimate additional Cases, we need to estimate the positivity level on the incremental tests. Generally speaking, when the number of tests goes up, you expect the positivity level to go down somewhat, on the theory that the sickest people tend to get tested first, so that additional tests hit a slightly healthier population.
In Florida’s case, this hasn’t always been true. Florida allocates tests not by need, but by county population. This means that tests don’t flow preferentially to the counties with the highest infection rate. Indeed, in a recent county level analysis, we discovered that the counties with the lowest Positivity Rate were actually testing more than the counties with the highest rates. (see Table 2 in this post). Were Florida to increase testing and change its allocation policies so that counties with higher positivity rates received more tests, there’s a reasonable chance Florida’s average Positivity rate would actually increase even as testing counts increased.
Nevertheless, we think it’s unlikely that Florida would change its allocation methods to one that would increase reported case counts. As a result, we decided to assume a similar allocation based on county population alone, and estimated the Positivity as as shown in Figure 2.
The Scenario Estimate in Figure 2 is calculated as follow:
- The original tests are carried forward at the observed Positivity rate
- Incremental tests within the scenario are estimated at a lower Positivity rate that decreases 10% for each 10% increment over the number of actual tests. In other words, the first 10% of incremental tests (as a % of actual tests for the week) are carried forward at 90% of the observed Positivity rate, the next 10% at 80%, and so forth.
As shown in Figure 3, for the week ending 8/28, the state actually tested only 180,645 people (with a reported Positivity Rate of 12.5%). In our scenario, we “tested” an additional 276,626 and calculated an incremental Positivity rate of 8.0% using our formula. This converted to a total of 459,151 tests at an average positivity rate of 9.8%.
Overall, in the 7 weeks since the peak week of July 17, the State of Florida reported 318,626 cases, instead of the 463,454 we estimated would have been generated had testing levels remained constant. This means that Florida under-reported new cases by more than 46%.
Based on this analysis, there remains some improvement in Florida’s situation. But it’s not nearly as large as normally reported. Note that the lower case count will ripple into other parts of the reporting. In particular, deaths resulting from cases that are not confirmed will NOT be reported in Florida’s official COVID counts, as Florida (contrary to CDC recommendations) does not report probable COVID deaths.
