The age-specific death rates are
calculated separately for separate groups of data which are believed to have
different mortality rates (e.g. males and females, and perhaps type of
residence, if data is available separately for these groups) from which one can
calculate the probability of surviving to each age.

The probability of surviving from
age

*x*to age*x+n*is denoted as_{n}p_{x}and the probability of dying during age*x*(i.e. between ages*x*and*x*+1) is denoted q_{x}. For example, if 5% of a group of people alive at their 50th birthday die before their 51^{st}birthday, then the age-specific death probability at age 50 would be 5%.
The life expectancy at age

*x*, denoted as*e*_{x}, is calculated as below. This is the expected number of complete years lived. Life expectancy is by definition an arithmetic mean of number of years of life remaining at a given age.
By using the theory of Expectation in the Theory
of Probability, it
is calculated as sum of the product of n
,

It is important to note that this statistic is
usually based on past mortality experience, and assumes that the same
age-specific mortality rates will continue into the future. Thus such life
expectancy figures need to be adjusted for temporal trends before calculating
how long a currently living individual of a particular age is expected to live. _{n}p_{x}and q_{x+t }; the summation is to from n = 0 to ∞.
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