Every piece of data we currently have about the novel coronavirus is imperfect and incomplete.

Almost three weeks after the first confirmed case of community spread—a patient who had not traveled anywhere with known cases or had contact with anyone known to be infected— the Centers for Disease Control and Prevention still is not publishing state-level data about how many people have been tested for the virus.

As a result, The Atlantic has been maintaining the Covid Tracking Project, the only source of state-level testing data through time, in collaboration with the data scientist Jeff Hammerbacher and a team of volunteers. That work has taught us how to read the different types of numbers that are trickling out of local, state, and federal agencies. The project has been able to track just over 8,000 people tested in the United States, where there are more than 1,000 cases of COVID-19, the disease caused by the coronavirus. Our number, too, is imperfect, but it allows us to better understand the scale of the country’s response to this pandemic.

These numbers, if anything, are an undercount, the magnitude of which is unknown. The extremely limited testing capacity of the U.S., combined with restrictive testing criteria, has curtailed the number of people who should be tested. How dramatic is the undercount? One analysis from Cedars-Sinai suggested that 9,000 people had the virus already on March 1. In Seattle alone, a separate analysis of viral genomes suggests that there may have been 600 cases on March 1 in that city alone, comparable to the number of cases in the city of Wuhan, China, the putative epicenter of the pandemic, on January 1.