The average expectation of life at birth is a good measure of the level of mortality because it is not affected by the age structure of the population.

This measure is derived from the life table which is constructed to summarise the mortality experience of a single hypothetical generation or a cohort of people subject to a set of constant age specific death rates throughout its lifetime.

The term “average expectation of life” represents the average number of years of life which a cohort of new-born babies (that is, those born in the same year) may be expected to live if they are subjected to the risks of death at each age according to the age specific mortality rates prevailing in the country at the time of which the measure refers.

Though the average expectation of life at birth is a measure which is rather complicated to calculate, it is most easily understood by the common man. It is, therefore, widely used to compare mortality levels and analyse mortality trends in different countries.