Population Pyramids, Population Projections and Life Expectancy
Population pyramids, projections and life expectancy summarise population structure and future needs. They help anticipate demand for maternity care, child health, schools, employment, chronic-disease services and care of older adults.
Learning objectives
- Construct and interpret an age-sex population pyramid.
- Differentiate expansive, stationary and constrictive shapes.
- Explain cohort patterns caused by fertility, mortality and migration.
- Describe population projection methods and assumptions.
- Define life expectancy and explain basic life-table concepts.
Population pyramid
A population pyramid is a paired horizontal bar chart showing the percentage or number of males and females in successive age groups. Age is arranged vertically, youngest at the base; males are conventionally displayed on the left and females on the right.
Steps in construction
- Group the population into equal age intervals, commonly five-year groups.
- Separate males and females.
- Convert counts to percentages if populations will be compared.
- Use the same horizontal scale on both sides.
- Label age groups, sex, units, place and year.
- Plot horizontal bars from the central axis.
Major pyramid shapes
| Shape | Features | Interpretation |
|---|---|---|
| Expansive | Broad base, rapidly narrowing sides | High fertility, youthful population and potential rapid growth |
| Stationary | Similar width through many younger and middle ages | Low fertility and mortality with slow or stable growth |
| Constrictive | Narrow base relative to middle ages | Low fertility, ageing and possible future decline |
Reading irregularities
Bulge
May reflect a previous baby boom, improved survival or immigration of a cohort.
Notch
May indicate war, famine, epidemic, low fertility or cohort emigration.
Sex imbalance
Can result from migration, conflict, occupation-specific movement or differential mortality.
Older female excess
Often reflects longer female survival and male mortality differences.
Interpretation rule
A pyramid suggests hypotheses but does not prove the cause. Confirm patterns using fertility, mortality, migration and historical data.
Health-planning uses
- Broad base: high demand for maternity, newborn, immunisation and paediatric services.
- Large adolescent cohort: need for schools, sexual and reproductive health, mental-health and employment services.
- Growing working-age group: opportunity for a demographic dividend if jobs and skills exist.
- Ageing population: increasing NCD, disability, rehabilitation and long-term-care needs.
- Migration bulges: occupational health, housing and mobile service delivery.
Population projections
A population projection estimates future size and composition using stated assumptions. It is not a prediction of certainty; it shows what would happen if assumptions occur.
Main methods
| Method | Description | Best use |
|---|---|---|
| Arithmetic | Adds a constant number each period | Short-term stable absolute growth |
| Geometric/exponential | Applies a constant percentage growth rate | Simple total-population projections |
| Cohort-component | Advances age-sex cohorts while applying fertility, mortality and migration assumptions | Detailed national and subnational projections |
Cohort-component steps
- Start with a base population by age and sex.
- Apply survival rates to advance cohorts.
- Estimate births using age-specific fertility rates.
- Apply infant and child survival to new births.
- Add immigrants and subtract emigrants by age and sex.
- Repeat for each projection interval.
Projection scenarios
Low, medium and high scenarios reflect different fertility, mortality or migration assumptions. Planners should compare scenarios and update projections after new census or survey data.
Simple projection example
A population of 250,000 grows by 2.5% annually. After three years:
P3 = 250,000 × (1.025)3 ≈ 269,221.
This simple estimate does not show age composition and assumes constant growth.
Projection errors
- Incorrect base population
- Unexpected migration, conflict or epidemic
- Rapid fertility or mortality change
- Boundary changes
- Long projection horizon
- Failure to update assumptions
Life expectancy
Life expectancy at birth is the average number of years a newborn would live if current age-specific mortality rates continued throughout life. It is a period summary of current mortality, not a guaranteed lifespan for an individual.
Life expectancy at age x is the average remaining years for a person who has already reached age x under current mortality conditions.
Life table concepts
| Symbol/concept | Meaning |
|---|---|
| lx | Number from a hypothetical starting cohort surviving to exact age x |
| dx | Number dying between age x and the next age interval |
| qx | Probability of dying within the interval |
| Lx | Person-years lived by the cohort in the interval |
| Tx | Total person-years remaining above age x |
| ex | Life expectancy at age x, calculated as Tx ÷ lx |
Types of life tables
- Period life table: uses age-specific mortality rates from a particular period.
- Cohort life table: follows an actual birth cohort through time.
- Complete life table: uses single-year age intervals.
- Abridged life table: uses broader intervals, commonly five-year groups.
Factors affecting life expectancy
Increase
- Lower infant and maternal mortality
- Vaccination and infection control
- Improved nutrition, education and sanitation
- Effective NCD and injury prevention
- Accessible quality healthcare
Decrease
- Epidemics, conflict and disasters
- High child and maternal mortality
- Poverty and unsafe environments
- Injuries and harmful substance use
- Weak access to effective care
Life expectancy versus related measures
- Healthy life expectancy: expected years lived in full health after accounting for disability.
- Median age at death: middle age among observed deaths; not equal to life expectancy.
- Maximum lifespan: biological upper limit, not a population average.
- Survival rate: probability of surviving a defined interval or condition.
Common interpretation errors
- Assuming every newborn will die at the reported life expectancy age.
- Comparing pyramid widths when different scales are used.
- Calling projections exact forecasts.
- Ignoring migration when interpreting cohort bulges.
- Comparing total counts instead of percentages between different-size populations.
Exam-focused summary
- Population pyramids display age-sex structure and reveal growth patterns.
- Expansive pyramids have broad bases; constrictive pyramids have narrow bases.
- Cohort-component projection separately models fertility, mortality and migration.
- Projection uncertainty increases with time and unstable assumptions.
- Life expectancy summarises current age-specific mortality rates.
Review questions
- Describe the steps for constructing a population pyramid.
- Compare expansive, stationary and constrictive shapes.
- Explain four causes of bulges or notches.
- Outline the cohort-component projection method.
- Define life expectancy at birth and at age x.
- Explain six basic life-table columns.
References and further reading
- United Nations Population Division: Methodology.
- WHO indicator metadata.
- Clinical Medicine Year 2 curriculum.
Educational use only. Use current UBOS and United Nations estimates and clearly state assumptions.
