At it's core, the study of mortality is based on a simple ratio - the number of deaths, D, divided by the population exposed to the risk of death, E:

mortality rate = D / E

While this seems simple and straightforward, there are important subtleties concerning the quality of the data used.  One of these is illustrated by a series of revelations about unreported deaths in Japan, where a recent audit has cast doubt on the reliability of Japanese population statistics for the elderly.  Unreported deaths reduce mortality rates both by under-counting deaths and by inflating the population at risk.  Time will tell if Japan's famously low mortality rates have been under-stated as a result.

The top line of a mortality rate…