Data definitions and comparison rules
Demographic Data Methodology
Demographic indicators are published on different dates, under different definitions and through different statistical systems. This methodology explains how values are selected, labeled and compared across country profiles.
Data Selection Order
- Official national statistics are used for national census counts, resident population estimates, births, deaths and migration flows when a current release is available.
- International datasets are used for harmonized cross-country series, long historical series and projections.
- The reference year, measurement type and source are kept with each indicator.
- A value is left unavailable when a suitable source cannot be identified.
Census Count
A census count records the population covered by a census operation at a defined reference time. Coverage rules, usual-residence criteria and publication stages vary by country.
Mid-Year Estimate
A mid-year estimate represents population around the middle of a calendar year. It may be produced from national estimates or an internationally harmonized demographic model.
Resident Population Estimate
A resident estimate may be based on population registers, administrative records or post-censal estimation. Its reference date may be year-end, quarter-end or another official date.
Population Projection
A projection is a scenario based on assumptions about fertility, mortality and migration. It is not an observed count and may change when a dataset is revised.
Census Counts and Population Estimates
Census counts and population estimates answer related but different questions. A census may enumerate people at a fixed reference date, while a mid-year estimate adjusts population to another date and may incorporate births, deaths, migration and coverage adjustments. A direct numerical difference does not by itself show an error.
| Measure | Main purpose | Comparison rule |
|---|---|---|
| Census count | Population recorded by a census operation | Compare with another census only after checking reference date and residence rules |
| Resident estimate | Official population maintained between censuses | Compare when the reference date and residence concept match |
| Mid-year estimate | Population around the middle of the year | Preferred for harmonized annual international comparisons |
| Projection | Scenario for a future date | Compare only with the same projection revision and scenario |
Migration Stock and Migration Flow
Migration stock and migration flow are stored as separate indicators.
| Indicator | Meaning | Time basis |
|---|---|---|
| International migrant stock | People meeting the dataset's migrant definition who live in a country | Point in time |
| Immigration flow | People entering a country under the source definition | During a period |
| Emigration flow | People leaving a country under the source definition | During a period |
| Net migration flow | Immigration minus emigration, or the source's demographic balance estimate | During a period |
A country can have a large migrant stock and a negative annual net migration flow at the same time. The indicators must not be merged into a single migration value.
National and International Urban Definitions
Countries classify urban areas through national legal, administrative, population-size or settlement criteria. International series may harmonize national definitions or apply a common settlement framework. Two urban population shares can therefore differ without either value being incorrect.
- National urban values are labeled with the national definition or administrative category.
- International urban values are labeled with the dataset and reference year.
- Direct comparison is marked as limited when the definitions differ.
- Administrative categories such as province centers are not presented as identical to a harmonized urban series.
Reference Years and Comparable Values
The newest available value is not always the best value for a comparison. The comparison tool applies the following labels:
The reference year and measurement type match.
The measurement type matches and the reference years differ by no more than one year.
The years, definitions or measurement types differ.
One or both countries do not have a usable value.
Data Revisions
Population series may change after a census, register update, methodological revision or international dataset revision. A revised value replaces the displayed value only after the source, reference year and measurement type have been checked.
- The dataset release or revision should be recorded when available.
- Historical values may change when a source reconstructs an earlier series.
- A page date should not imply that every indicator was measured in that year.
- Projection revisions should not be mixed within one comparison table.
Age Structure, Fertility and Life Expectancy
Broad age groups normally use ages 0–14, 15–64 and 65+. Values from a different grouping are labeled separately. Total fertility rate is reported as births per woman under the source's period definition. Life expectancy at birth represents mortality conditions in the stated period and is not an estimate of the age an individual will necessarily reach.
Sources, Missing Data and Corrections
Country profiles identify national statistics offices, census agencies, international organizations and statistical databases used for each table. Source links should point to the release or dataset rather than a general institutional home page whenever possible.
Missing indicators remain unavailable. Values are not inferred from neighboring countries, unrelated years or unsourced summaries. Corrections are recorded through the site's corrections policy.