The number of further years of life a person can expect at a given age. The measure is calculated from a life-table
, and since it is expressed as an average for persons of that age and sex in a country, depends upon prevailing (current) levels of mortality at different ages within the population or sub-population to which the individual belongs. Since, in all societies, mortality
rates between birth and the first birthday tend to be particularly high, life-expectancies at birth are usually considerably lower than life-expectancies at year one. As one would expect from variations in mortality rates between countries, life-expectancies also vary considerably, being around 30 to 40 years at birth in certain developing countries, and reaching 75 and over for women in the major Western industrialized societies. Life-expectancy at birth is a widely used indicator of health standards and social and economic living standards. It is also possible to derive life-expectancies for different sub-groups of populations, for example different social classes, providing mortality rates are known.