A massive statewide surge of 55,500 tests on May 20 had no effect on the number of positive tests.
The graphic above plots two data series on coronavirus testing in the entire state of Florida from April 19 to July 18, based on the nationwide monitoring efforts of the COVID Tracking Project. The first data series, rendered as the connected blue line and measured on along the left-hand vertical axis, shows the total number of test results reported on each day. The second data series, rendered as the connected burgundy line and measured along the right-hand vertical axis, shows the number of positive test results on the same day. The two data series are measured on different scales – with the positive tests magnified four-fold compared to total tests – so that both series can be readily visualized on the same graph.
The question posed by the graphic appears straightforward: Did the number of COVID-19 infections, as captured in the positive-test data series, push ahead the total amount of testing? Or did the total number of tests performed pull on the detected number of COVID-19 infections? Put more succinctly, did the movements in the burgundy line cause the movements in the blue line? Or was the causation the other way around?
In reality, the question is improperly framed. There is no reason that the direction of causation goes only one way. It’s entirely possible that the forces of both push and pull have been operating simultaneously. On the push side, it is possible that a surge in infections has increased the demand for testing. On the pull side, it is possible that enhanced testing has resulted in the detection of COVID-19 infections that would otherwise have gone undetected. The more precise question is: Which of the two forces, if any, was more important?
To resolve the question, we would ideally perform two experiments, which we can describe in the context of the push-pull diagram above. Experiment #1: We’ll have the pusher let up and have the puller keep pulling, and then see if the pusher and cargo move. To make sure, we could have the puller exert maximum effort while the pusher just holds onto the cargo and takes it easy. Experiment #2: We’ll have the pusher exert maximum force while the puller goes along for the ride, and once again see whether the puller and cargo budge.
Fortunately, Florida has performed both experiments for us. To see how, we’ve annotated the opening graphic. On May 14, 2020, Gov. Ron DeSantis issued Executive Order 20-123, Full Phase 1: Safe. Smart. Step-by-Step. Plan for Florida’s Recovery. The order, which took effect on May 18, allowed restaurants, retail establishments, and gyms to operate at 50 percent capacity, opened professional sports events and training camps, and permitted amusement parks and vacation rentals to operate subject to prior approval.
As part of the Full Phase 1 effort, the state massively expanded its COVID-19 testing capacity. On May 19, the day after Full Phase 1 went into effect, the governor held a press conference discussing in part the state’s expanded COVID-19 testing efforts. As documented in the Florida Department of Health press release for that day, the governor took the following steps:
- FLNG [Florida National Guard] has expanded its support to mobile testing teams and the community-based and walk-up test sites. To date, the FLNG has assisted in the testing of more than 227,000 individuals for the COVID-19 virus.
- In an effort to increase testing, Governor DeSantis has directed Surgeon General Dr. Scott Rivkees on an emergency temporary basis to allow licensed pharmacists in Florida to order and administer COVID-19 tests.
- At the direction of Governor DeSantis, the state has established 15 drive-thru and 10 walk-up testing sites across the state, with more coming online. More than 100,000 people have been tested at these sites. Floridians can find a site near them here.
- At the direction of Governor DeSantis, AHCA [Agency for Health Care Administration] issued Emergency Rule 59AER20-1 on May 5 requiring COVID-19 testing by hospitals of all patients, regardless of symptoms, prior to discharge to long-term care facilities.
The graphic confirms a massive increase in testing after Full Phase 1 went into effect. The following day, on May 20, 2020, a total of 55,493 test results were reported, of which only 527 (or 0.95%) were positive and 54,966 were negative. (While the massive increase could have reflected the clearing of a huge backlog of recently performed but as yet unreported tests, we still can’t get around the fact that less than 1 percent of those tests were positive.) The green dashed lines in the graphic highlight the step-up in testing. Before Full Phase 1, from April 19 through May 17, an average of 13,975 test results were reported daily. In the month immediately after Full Phase 1 went into effect, from May 28 through June 24, an average of 26,751 tests results were reported daily. That’s nearly double the amount of testing.
When testing massively increased on May 20, positive tests did not change. When the puller pulled with maximum effort, the pusher and cargo didn’t budge. During the month after Full Phase 1, while total testing on average had nearly doubled, positive tests began to move upward but only with a delay of couple of weeks. As a result of the state’s Full Phase 1 reopening, the pusher was starting to push.
As documented in Data From the COVID-19 Epidemic in Florida Suggest That Younger Cohorts Have Been Transmitting Their Infections to Less Socially Mobile Older Adults, the initiation of Full Phase 1 was soon followed by an increase in COVID-19 incidence. In parallel with gains in social mobility indicators – such as the number of diners at sit-down restaurants – COVID-19 infections surged among younger adults and also increased substantially among older individuals. By about June 25, as shown in the above graphic, COVID-19 infections had increased to the point where they were forcing further expansion of total testing.
The pusher was pushing hard, and the puller was going along for the ride.
Thus far, we have avoided the technical jargon of policy evaluation. For the record, we’ll translate here. In natural experiment #1, an exogenous increase in testing driven by an abrupt change in public policy had no observable effect on the number of positive tests. By contrast, in natural experiment #2, when the exogenous policy relaxed social distancing restrictions, enhanced social mobility and thus drove up COVID-19 infections, the observable effect on total tests was substantial.
The above graphic, which is directed to specialists in policy evaluation, communicates the same conclusions in the form of an X-Y plot. The vertical axis (or Y-axis, in mathematical jargon) measures the number of positive tests. The horizontal axis (or X-axis), measures the total number of tests. During the period from April 19 through June 24, 2020, as indicated by the green data points, the relation between positive tests (Y-axis) and total tests (X-axis) was basically flat. After the surge in COVID-19 cases, which we’ve pegged here as starting on June 25, there is a direct relationship between the two variables. Positive tests (the Y variable) have been driving up total tests (the X variable).
The evidence from Florida strongly supports the conclusion that the recent surge in COVID-19 cases has pushed up the total amount of testing. The observed rise in testing did not pull along the number of positive tests.
Acknowledgments: Thanks to A. Marm Kilpatrick for pointing out that the massive bump in reported tests on May 20 could have represented the clearing of a huge backlog of recently performed but as yet unreported tests.