In an ideal world, we would know exactly how many COVID-19 infections there are in the United States at a given time—a measurement known as disease prevalence, which is usually portrayed as a percentage of the population.
Although it’s hard to measure COVID-19 prevalence, we can look at the number of “confirmed cases,” a figure that corresponds to the number of viral diagnostic tests that come back positive. By dividing this number by the total number of reported tests and multiplying the result by 100, we get a test positivity rate, also known as percent positivity. US states and territories report tests using several different units, so it’s important to use the same unit in the numerator (positive tests) and denominator (total tests) of this equation.
