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National Institute for Communicable Diseases/DATCOV, South Africa
The Delphi Group at Carnegie Mellon University U.S. COVID-19 Trends and Impact Survey, in partnership with Facebook
Cases and 21-day Slope
The light blue in these is a basic epidemic curve for the given area. It represents the number of cases on a given day on the left y-axis. The x-axis is simply the date. The dark blue line following the curve is simply the seven day running average, which is how much of the media shows the data.
The red line in this particular graph is the first derivative of the cases. It finds the average slope (the total number of cases/the number of days) over the past three weeks and is measured on the right y-axis. There is also a dotted black line representing zero for this particular part of the graph. When the red line is at zero, cases are remaining flat. The distance the the red line gets above or below zero represents the rate at which cases are increasing or decreasing.
A good analogy to understand this is that if the red line is very high, it’s similar to someone flooring the accelerator of a car. The opposite is true if the red line is a long distance below zero. It’s equivalent to someone stomping on the brake. If the red line were just a little above or below zero, they are either accelerating or braking slowly.
On the right side you will see the red line extending out past the epidemic curve below it. One of the benefits of using a first derivative is that it can be used to project a little bit forward where the pandemic is headed in a particular area. I’ve found that it’s reasonably accurate about 10 days out, but obviously the margin of error gets wider the further that data is projected out from the present.
PHOTO: Colorized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) grown in MDCK cells (seen in green).
Photo Courtesy CDC-PHIL