Changes

Demographers can measure or estimate the life expectancies of a birth cohort at any age of the cohort. Demographers use the symbol e₀ for life expectancy at birth (age 0), e₂₀ for life expectancy at age 20, etc. Exact calculation requires a [[life table]] that shows what proportion of people at a given age die that year. When data is incomplete, it is possible to use "model life tables" to estimate life expectancy.  When no age is specified, e₀ is meant.

Demographers can measure or estimate the life expectancies of a birth cohort at any age of the cohort. Demographers use the symbol e₀ for life expectancy at birth (age 0), e₂₀ for life expectancy at age 20, etc. Exact calculation requires a [[life table]] that shows what proportion of people at a given age die that year. When data is incomplete, it is possible to use "model life tables" to estimate life expectancy.  When no age is specified, e₀ is meant.

::''Click Gallery tab above to see an example of a life expectancy table, showing life expectancy at birth, at 65 years of age, and at 75 years of age, by race and sex, for the United States, for selected years from 1900–2004, which thus show trends for the 20th century.''

[[File:New-meds.jpg|thumb|350px|new drugs prolong life]]

In recent years life expectance has gone up. In the Third World, the cause is lowering infant mortality, plus antibiotics for all ages. In developed country, 40% of the rise is due to new drugs.

==Historical trends==

==Historical trends==