Preliminary research by mathematical modelers in the Centre for the Mathematical Modelling of Infectious Diseases at the London School of Hygiene & Tropical Medicine found the U.S.’s case count likely represented just 14% to 19% of actual infections that produced symptoms. (By contrast, South Korea had identified between 53% and 90% of total cases in the country, the researchers said.)

Local governments vary widely in how they collect and disclose their testing results, which can affect statewide totals. For instance, most states report just positive or negative test results, while others also report tests for which results are pending.

There is also the issue of tests done by private laboratories, working under contracts with hospitals and local health departments. Some states don’t report them or do so inconsistently. Until that data is more complete, state comparisons will be impossible and national figures will be potentially misleading.

Experts say that the goal should be broad testing throughout the population to get an accurate picture of who is infected. Iceland has tested a broad cross-section of its population and found that middle-age people make up 40% of those testing positive. In the U.S., where early testing has focused on people who show symptoms, the age distribution has skewed toward older groups.