Seeing exponential growth for what it is

Understanding the spread of SARS-CoV-2, the virus responsible for COVID-19, requires understanding nonlinear growth, according to Jeffrey M. Zacks, associate chair and professor of psychological and brain sciences in Arts & Sciences, and of radiology at the School of Medicine, at Washington University in St. Louis.

Whereas linear growth is intuitive, nonlinear growth is not. People’s predictions for nonlinear patterns tend to be closer to linear projections, assuming that future growth will be similar to that of the past. For a pandemic, this can lead to dangerous underestimations in the time to reach a critical value.

Most physical phenomena, such as the accumulation of snow or spread of a spill, have linear growth rates or rates that only increase slowly. Exponential growth, in which the rate of growth itself rapidly increases, may be less frequently observed, in part because such growth often quickly ceases after reaching some boundary condition. Pandemics such as COVID-19 grow exponentially, which can challenge understanding.

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