"Scientists combine individual pieces of evidence about each of these questions to build a picture of the death rate. For example, they estimate the proportion of cases with mild symptoms from small, defined groups of people who are monitored very tightly, like people on repatriated flights. But slightly different answers from those pieces of evidence will add up to big changes in the overall picture. If you just use data from Hubei, where the death rate has been much higher than elsewhere in China, then the overall death rate will look much worse. So scientists give a range as well as a best current estimate. But even that doesn't tell the full story because there is no single death rate..."