Ah....
"Mortality rates are usually calculated on patients whose outcome is known," said Dr. Henry Niman, a surgery instructor at the Harvard/Massachusetts General Hospital. "(The CDC's number [4%] ) would be accurate only if all patients hospitalized survived, which has not been the history of the disease in the more heavily affected areas."
Instead, some say the rate should be calculated by dividing the number of deaths by the sum of the number of deaths plus the number of people who recovered, which would exclude those people who are still sick, resulting in a higher death rate.
According to Niman's math, the death rate in Hong Kong would be 25 percent, in Canada 21 percent, in Singapore 15 percent and in Vietnam 10 percent. In China, the death rate would be only 5 percent.
From:
http://www.wired.com/news/medtech/0,1286,58552,00.html