How do you calculate life expectancy

Life expectancy has doubled in all world regions. What does this mean exactly? In this section, we try to fill this gap.

By definition, life expectancy is based on an estimate of the average age that members of a particular population group will be when they die. One important distinction and clarification is the difference between cohort and period life expectancy. The cohort life expectancy is the average life length of a particular cohort — a group of individuals born in a given year.

You can think of life expectancy in particular year as the age a person born in that year would expect to live if the average age of death did not change over their lifetime. It is of course not possible to know this metric before all members of the cohort have died. Because of that statisticians commonly track members of a particular cohort and predict the average age-at-death for them using a combination of observed mortality rates for past years and projections about mortality rates for future years.

An alternative approach consists in estimating the average length of life for a hypothetical cohort assumed to be exposed, from birth through death, to the mortality rates observed at one particular period — commonly a year. Period life expectancy estimates do not take into account how mortality rates are changing over time and instead only reflects the mortality pattern at one point in time.

Because of this, period life expectancy figures are usually different to cohort life expectancy figures. Since period life expectancy estimates are ubiquitous in research and public debate, it is helpful to use an example to flesh out the concept.

You can hover the mouse over a country to display the corresponding estimate. For Japan, we can see that life expectancy in was This means that a hypothetical cohort of infants living through the age-specific mortality of Japan in could expect to live But if life expectancies are increasing the reality for a cohort born then is that the cohort life expectancy is higher than that period life expectancy.

In general, the commonly-used period life expectancies tend to be lower than the cohort life expectancies, because mortality rates were falling over the course of modern development. Whenever mortality rates are falling then the period life expectancy is lower than the life expectancy of the cohort born then.

An important point to bear in mind when interpreting life expectancy estimates is that very few people will die at precisely the age indicated by life expectancy, even if mortality patterns stay constant.

For example, very few of the infants born in South Africa in will die at Most will die much earlier or much later, since the risk of death is not uniform across the lifetime.

Life expectancy is the average. In societies with high infant mortality rates many people die in the first few years of life; but once they survive childhood, people often live much longer.

Indeed, this is a common source of confusion in the interpretation of life expectancy figures: It is perfectly possible that a given population has a low life expectancy at birth, and yet has a large proportion of old people. Given that life expectancy at birth is highly sensitive to the rate of death in the first few years of life, it is common to report life expectancy figures at different ages, both under the period and cohort approaches.

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