There is clearly some group of Americans who have the coronavirus but who don’t show up in official figures. Now, using a statistic that has just become reliable, we can estimate the size of that group — and peek at the rest of the iceberg. According to the Tracking Project’s figures, nearly one in five people who get tested for the coronavirus in the United States is found to have it. In other words, the country has what is called a “test-positivity rate” of nearly 20 percent.