With coronavirus testing, many governments have used the percentage of tests that came back positive over time to gauge progress and decide whether or not it’s time to reopen. To calculate percentage, they divide confirmed cases by total tests. The denominator — total tests — often comes from the CDC, which apparently hasn’t done a good job calculating that denominator, because not all tests are the same.

Alexis C. Madrigal and Robinson Meyer for The Atlantic:

Mixing the two tests makes it much harder to understand the meaning of positive tests, and it clouds important information about the U.S. response to the pandemic, Jha said. “The viral testing is to understand how many people are getting infected, while antibody testing is like looking in the rearview mirror. The two tests are totally different signals,” he told us. By combining the two types of results, the CDC has made them both “uninterpretable,” he said.

Oh.

Tags: Atlantic, CDC, coronavirus, denominator, testing