The death rate is not calculated on the basis of who has been tested, but on confirmed cases...
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Mama always said Assumption is the Mother of all Foul Ups. She actually used stronger language. Seriously, for COVID-19 all the data needed for the correct denominator is not yet known. Most of the missing data are the number of infected but not yet confirmed. That missing data will be known, in a statistically valid manner, in the coming weeks and will yield a HUGELY LOWER death rate.
The notion that somehow, one case in China has infected billions of people in just 4 months is not mathematically supportable. It is also not biologically plausible.