Measurement of Population Size and Composition
Population measurement describes how many people live in an area, where they live and how they are distributed by characteristics such as age and sex. Accurate measurement supplies denominators for rates and guides health, education, housing and economic planning.
Learning objectives
- Define population size, density, distribution and composition.
- Estimate populations between and beyond census years.
- Calculate sex, dependency and age-group ratios.
- Interpret age-sex composition for health planning.
- Recognise common errors in population data.
Population size
Population size is the total number of people residing in a defined area at a specified reference time. The definition must state geographical boundaries, usual-residence rules and the reference date.
De facto and de jure enumeration
| Approach | Who is counted? | Strength |
|---|---|---|
| De facto | People present in the area on census night | Reduces recall about usual residence but may distort mobile populations |
| De jure | People according to usual place of residence | Better for local service planning but requires clear residence rules |
Methods of obtaining population size
- Census count: most complete periodic enumeration.
- Population register: continuously updated administrative record.
- Survey estimate: extrapolates from a representative sample.
- Demographic balancing equation: updates the previous population using births, deaths and migration.
- Projection: applies assumptions about future fertility, mortality and migration.
Intercensal estimation
When two census counts are known, population between them may be estimated using linear or exponential assumptions.
Arithmetic growth
Pt = P0 + kt, where k is the average absolute increase per year.
Geometric or exponential growth
Pt = P0(1 + r)t, where r is the annual growth rate expressed as a decimal.
Worked estimate
A district population is 100,000 and grows at 3% annually. After two years:
100,000 × (1.03)2 = 106,090.
Projections are estimates, not exact counts; uncertainty increases when migration or growth assumptions change.
Population distribution
Distribution describes spatial arrangement, including urban-rural residence, regional concentration and settlement pattern.
Population density
Arithmetic density = Total population ÷ Land area, usually expressed as people per square kilometre.
High average density does not prove overcrowding because people may cluster in only part of the land. Local maps and settlement data improve interpretation.
Population composition
Age
Determines demand for maternal care, immunisation, schools, employment, chronic care and elderly services.
Sex
Supports reproductive-health planning and analysis of sex-specific risks and survival.
Socioeconomic
Education, occupation, income and housing help describe opportunities and vulnerability.
Other characteristics
Marital status, disability, ethnicity, migration status and household structure may guide equitable services.
Age structure
Common broad groups are:
- Children: 0–14 years
- Working-age population: 15–64 years
- Older population: 65 years and above
Exact groupings may vary by programme. Maternal health may use women of reproductive age; immunisation uses specific childhood ages.
Sex ratio
Sex ratio = Number of males ÷ Number of females × 100
A sex ratio of 96 means 96 males per 100 females. Interpret by age because migration and mortality can affect groups differently.
Dependency ratios
Dependency ratios compare groups conventionally considered dependent with the working-age population. They indicate potential social and economic support burden but do not measure actual employment or dependency.
- Child dependency ratio = Population aged 0–14 ÷ Population aged 15–64 × 100
- Old-age dependency ratio = Population aged 65+ ÷ Population aged 15–64 × 100
- Total dependency ratio = (Population 0–14 + population 65+) ÷ Population 15–64 × 100
Worked example
A population contains 40,000 children, 55,000 people aged 15–64 and 5,000 aged 65+.
- Child dependency = 40,000 ÷ 55,000 × 100 = 72.7.
- Old-age dependency = 5,000 ÷ 55,000 × 100 = 9.1.
- Total dependency = 45,000 ÷ 55,000 × 100 = 81.8 dependants per 100 working-age people.
Other composition measures
| Measure | Formula or meaning | Use |
|---|---|---|
| Proportion under five | Under-five population ÷ total population × 100 | Child-health and immunisation planning |
| Proportion urban | Urban population ÷ total population × 100 | Urban services, housing and transport |
| Average household size | Household population ÷ number of households | Housing, water and social planning |
| Literacy rate | Literate persons in defined age group ÷ population of same group × 100 | Education and health communication |
| Labour-force participation | Labour force ÷ working-age population × 100 | Economic and dependency analysis |
Health-planning applications
- Estimate pregnancies, births and vaccine target populations.
- Calculate disease, mortality and service-coverage rates.
- Plan facilities according to population density and travel distance.
- Forecast age-specific medicines and staffing needs.
- Identify rapidly growing urban settlements and mobile populations.
- Monitor inequity using disaggregated population denominators.
Data-quality checks
| Problem | Clue | Response |
|---|---|---|
| Age heaping | Unusual preference for ages ending in 0 or 5 | Improve interviewing and validate dates |
| Sex-selective undercount | Implausible sex ratios | Review field coverage and migration |
| Boundary changes | False apparent growth or decline | Use consistent geography |
| Duplicate enumeration | Inflated counts in mobile groups | Apply residence rules and de-duplication |
| Outdated projections | Coverage over 100% or implausible rates | Update using recent census and local evidence |
Exam-focused summary
- Population size requires a defined place and reference time.
- Composition includes age, sex and socioeconomic characteristics.
- Density is population divided by area.
- Dependency ratios use conventional age groups and do not equal actual economic dependency.
- Population estimates and projections must state assumptions and uncertainty.
Review questions
- Differentiate de facto and de jure census approaches.
- Calculate a two-year population projection at a given growth rate.
- Calculate sex ratio and total dependency ratio.
- Explain five health-planning uses of population composition.
- Describe five common errors in population data.
References and further reading
Educational use only. Use current UBOS definitions, boundaries and population estimates for applied work.
