Epidemiology · Reference
What is life expectancy?
Life expectancy is the average number of years a person can expect to live, calculated from the death rates of a population using a life table. It is a key demographic indicator of population health, and is reported in period and cohort forms.
How life expectancy is calculated
Life expectancy is derived from a life table, which applies a population’s age-specific mortality rates to a hypothetical cohort and tracks how many would survive to each successive age. From this, the average number of years remaining at any given age is computed. The most cited figure is life expectancy at birth, the average lifespan implied by current death rates, but life expectancy can be calculated at any age (for example, remaining years of life at age 65). It is fundamentally a summary of mortality across all ages, condensed into a single, interpretable number.
Period versus cohort life expectancy
There are two distinct kinds. Period life expectancy applies the death rates observed in a single period (say one year) across all ages, as if a person experienced today’s age-specific mortality throughout their life. It is a snapshot of current mortality and the form most commonly reported.
Cohort life expectancy follows an actual birth cohort and uses the death rates they genuinely experience (and, for the future, are projected to experience) as they age. Because mortality has tended to improve over time, cohort life expectancy is usually higher than the period figure for the same birth year. Confusing the two leads to misinterpretation of trends.
A measure of population health
Life expectancy is one of the most widely used indicators of population health and development, allowing comparison across countries and over time. National statistical agencies such as the Office for National Statistics and international bodies including the World Health Organization and OECD publish it regularly. Related measures refine it: healthy life expectancy estimates the years lived in good health, combining mortality with morbidity, while gaps in life expectancy between social groups are a stark expression of health inequity.
Common misunderstandings
A frequent error is reading life expectancy at birth as the age at which people in a population typically die today, or as a forecast of an individual’s lifespan. In fact period life expectancy at birth reflects current death rates across all ages and is heavily influenced by mortality early in life; historically, high infant mortality pulled the figure down even though many who survived childhood lived to old age. It is a population-level statistical construct, not a personal prediction, and changes in it reflect shifts in mortality rather than any guarantee for a given person.
Key facts
At a glance
- Definition: Average years a person can expect to live
- Built from: Age-specific death rates in a life table
- Common form: Life expectancy at birth (current death rates)
- Period vs cohort: Snapshot of today vs an actual cohort’s lifetime
- Related: Healthy life expectancy (years in good health)
Common questions
FAQ
What is the difference between period and cohort life expectancy?+
Period life expectancy applies the death rates of a single period across all ages, giving a snapshot of current mortality, while cohort life expectancy follows an actual birth cohort using the death rates they genuinely experience over their lives. Because mortality has tended to improve, cohort figures are usually higher than period figures for the same birth year.
Is life expectancy a prediction of how long I will live?+
No. Life expectancy is a population-level statistical measure derived from current or projected death rates across all ages, not a forecast for any individual. Period life expectancy at birth in particular reflects today’s mortality across the whole age range, not the typical age of death for one person.
Why was historical life expectancy at birth so low?+
Life expectancy at birth is strongly affected by deaths in early life, so high infant and child mortality in the past pulled the figure down even though many people who survived childhood went on to live into old age. It is an average across all ages, not the typical age of death for survivors.
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