{
  "id": "life-expectancy",
  "title": "Life Expectancy",
  "lang": "en",
  "source_lang": "en",
  "summary": "Life expectancy is the average number of additional years a person would live under a specified pattern of age-specific mortality.",
  "category": "health",
  "subcategory": "public-health",
  "type": "concept",
  "aliases": [
    "Expectation of life"
  ],
  "generated_at": "2026-10-22",
  "model": "pro/gpt-6.1-sol",
  "url": "https://aiwiki.page/en/life-expectancy/",
  "translations": {
    "zh-Hans": {
      "url": "https://aiwiki.page/zh/life-expectancy/",
      "json": "https://aiwiki.page/zh/life-expectancy.json"
    }
  },
  "sections": [
    {
      "heading": "",
      "text": "Life expectancy is a statistical measure of the average number of additional years a person of a given age would live under a specified pattern of mortality rates. The most commonly reported form, life expectancy at birth, expresses the average lifespan implied by those rates. Widely used in demography and public health, it describes mortality conditions in a population rather than predicting the lifespan of a particular individual. Its interpretation depends especially on whether it is calculated for a fixed period or for an actual birth cohort. (ons.gov.uk)"
    },
    {
      "heading": "Period and cohort life expectancy",
      "text": "Period life expectancy applies the age-specific mortality rates observed during one year, or a group of years, to a hypothetical population throughout its lifetime. For example, a period estimate for 2020 combines mortality at infancy, adulthood, and old age during 2020, although the people observed at those ages belonged to different generations. It summarizes mortality conditions during that period; it does not assume that an actual generation will necessarily experience those conditions forever. Period estimates are useful for comparing mortality across places and over time. (ons.gov.uk)\n\nCohort life expectancy follows people born during the same specified period, usually a calendar year. It uses the mortality rates that this cohort experiences as it ages. A complete historical estimate requires observation until the cohort has died out; estimates for living cohorts therefore combine observed mortality with assumptions about future mortality. Cohort estimates can incorporate anticipated improvements or deteriorations, but consequently depend on projection assumptions. (ons.gov.uk)\n\nNeither measure is inherently interchangeable with the other. Period life expectancy answers a question about a fixed mortality schedule; cohort life expectancy answers a question about the lifetime experience of a generation. (ons.gov.uk)"
    },
    {
      "heading": "Calculation with life tables",
      "text": "Life expectancy is calculated using a life table, which translates age-specific probabilities of death into a pattern of survival. A table commonly begins with a hypothetical population of 100,000 births. For each age \\(x\\), it records quantities such as:\n\n- \\(q_x\\): the probability of dying before reaching age \\(x+1\\), conditional on surviving to age \\(x\\);\n- \\(l_x\\): the number surviving to exact age \\(x\\);\n- \\(L_x\\): the total person-years lived between ages \\(x\\) and \\(x+1\\);\n- \\(T_x\\): the total person-years remaining to be lived by those surviving to age \\(x\\).\n\nRemaining life expectancy is then\n\n\\[e_x=\\frac{T_x}{l_x}.\\]\n\nThus, \\(e_0\\) is life expectancy at birth, while \\(e_{65}\\) is the average number of additional years implied for someone who has reached age 65. Calculating person-years requires assumptions about when deaths occur within each age interval, with special treatment often used for infancy. (ssa.gov)\n\nComplete life tables use single-year age intervals; abridged tables group ages into broader intervals. The World Health Organization uses age- and sex-specific mortality estimates to construct life tables for international reporting. Where death registration is incomplete, mortality estimation may also draw on surveys, population censuses, and demographic models. Differences in data and methods can produce different estimates for the same population and year. (ons.gov.uk)"
    },
    {
      "heading": "Interpreting the measure",
      "text": "Remaining years versus expected age at death\n\nLife expectancy at a particular age refers to remaining years, not total age at death. Under the specified mortality schedule, the corresponding expected age at death is \\(x+e_x\\). It cannot generally be obtained by subtracting current age from life expectancy at birth: people who have survived to an older age have already passed the risks of dying at younger ages. (ons.gov.uk)\n\nFor illustration, a hypothetical population could have life expectancy at birth of 75 years and remaining life expectancy at age 65 of 20 years. The latter would imply an expected age at death of 85 among survivors to 65, without contradicting the birth estimate.\n\nAn average, not a maximum or a typical death age\n\nLife expectancy is an expected value, not a biological limit on human lifespan. It also differs from the median age at death, which divides life-table deaths into two equal halves, and the modal age at death, at which deaths are most concentrated. These measures respond differently to deaths at younger and older ages. (ons.gov.uk)\n\nThe importance of childhood mortality\n\nDeaths in infancy and childhood strongly affect life expectancy at birth because they remove many potential years of life. Consequently, a historically low birth estimate does not mean that most adults died near that age. Nevertheless, historical increases in life expectancy have not resulted solely from falling childhood mortality: survival has also improved during adulthood and old age. (ourworldindata.org)"
    },
    {
      "heading": "Historical change and population differences",
      "text": "Over the past two centuries, life expectancy has increased substantially across world regions, although progress has been uneven and interrupted. These changes accompanied improvements in living conditions, sanitation, nutrition, disease prevention, and medical treatment. Wars and epidemics, including HIV/AIDS and the COVID-19 pandemic, have produced major setbacks. Historical estimates are less certain where records are sparse. (ourworldindata.org)\n\nAn example of a sudden reversal appears in WHO estimates published in 2024: global life expectancy fell by 1.8 years between 2019 and 2021, reaching 71.4 years in 2021. This described a change in population mortality conditions, not an identical reduction in the remaining lifespan of every person. (who.int)\n\nDifferences within and between populations reflect more than access to medical treatment. Social determinants of health—including education, income, housing, working conditions, and access to essential resources—shape exposure to risks and opportunities for health. National averages can therefore conceal substantial inequalities between social groups and geographical areas. Such differences are not, by themselves, evidence of a single cause. (who.int)"
    },
    {
      "heading": "Healthy life expectancy",
      "text": "Healthy life expectancy adds information about health status to information about survival. WHO's healthy life expectancy measure, commonly abbreviated HALE, estimates the equivalent number of years expected to be lived in full health, allowing for time lived in less than full health because of disease or injury. Its calculation combines life-table information with estimates of ill health and its severity. (cdn.who.int)\n\nHALE is not interchangeable with every measure called “healthy life expectancy.” Other measures may use self-reported health or freedom from disability. Their definitions and methods must be checked before comparisons are made. Longer life expectancy alone does not establish how additional years are distributed between good and poor health. (cdn.who.int)"
    },
    {
      "heading": "Uses and limitations",
      "text": "Life expectancy informs population projections, pension planning, and health-policy assessment. Period estimates offer a standardized summary of mortality, while cohort estimates are more directly suited to questions about the future survival of particular generations. Neither provides an individualized medical prognosis. (ons.gov.uk)\n\nImportant limitations include incomplete death registration, inaccurate age reporting, small numbers of deaths, and uncertainty about mortality at very old ages. Cohort projections face the additional uncertainty of future mortality trends. Comparisons therefore require attention to the population covered, reference period, estimation method, and whether the figures are observed or projected. (cdn.who.int)\n\nAs a single average, life expectancy also omits much of the distribution of lifespans. Populations can differ in the concentration of deaths at particular ages even when their average longevity is similar. Measures of lifespan variation, median and modal death ages, and health expectancy provide information that the mean alone cannot supply. (ons.gov.uk)"
    }
  ],
  "links": [
    "mortality-rate",
    "demography",
    "public-health",
    "world-health-organization",
    "population-census",
    "expected-value",
    "hiv-aids",
    "pandemic",
    "social-determinants-of-health",
    "education",
    "disability"
  ],
  "sources": [
    {
      "title": "Period and cohort life expectancy explained - Office for National Statistics",
      "url": "https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/lifeexpectancies/methodologies/periodandcohortlifeexpectancyexplained"
    },
    {
      "title": "Guide to interpreting past and projected period and cohort life tables - Office for National Statistics",
      "url": "https://www.ons.gov.uk/peoplepopulationandcommunity/healthandsocialcare/healthandlifeexpectancies/methodologies/guidetointerpretingpastandprojectedperiodandcohortlifetables"
    },
    {
      "title": "Definitions of Life Table Functions",
      "url": "https://www.ssa.gov/oact/Downloadables/LifeTableDefinitions.pdf"
    },
    {
      "title": "WHO estimates of life expectancy and healthy life expectancy",
      "url": "https://cdn.who.int/media/docs/default-source/gho-documents/global-health-estimates/ghe2021_lifetable_methods.pdf"
    },
    {
      "title": "Mortality in England and Wales - Office for National Statistics",
      "url": "https://www.ons.gov.uk/peoplepopulationandcommunity/birthsdeathsandmarriages/lifeexpectancies/articles/mortalityinenglandandwales/pastandprojectedtrendsinaveragelifespan"
    },
    {
      "title": "Life Expectancy",
      "url": "https://ourworldindata.org/life-expectancy"
    },
    {
      "title": "COVID-19 eliminated a decade of progress in global level of life expectancy",
      "url": "https://www.who.int/news/item/24-05-2024-Covid-19-eliminated-a-decade-of-progress-in-global-level-of-life-expectancy"
    },
    {
      "title": "Social determinants of health",
      "url": "https://www.who.int/health-topics/social-determinants-of-health"
    },
    {
      "title": "World Report on Social Determinants of Health Equity",
      "url": "https://www.who.int/teams/social-determinants-of-health/equity-and-health/world-report-on-social-determinants-of-health-equity"
    },
    {
      "title": "Trends and disparities in life expectancy and healthy life expectancy",
      "url": "https://cdn.who.int/media/docs/default-source/gho-documents/world-health-statistic-reports/worldhealthstatistics_2022.pdf"
    }
  ],
  "license": "https://creativecommons.org/licenses/by/4.0/"
}