1. Introduction

In Northern Ireland the Northern Ireland Statistics and Research Agency (NISRA) produce annual estimates of the resident population. The population estimates refer to the number of people in the population at 30 June each year, the statistics are therefore often referred to as the mid-year estimates.

The population statistics refer to the usually resident population. The definition of usual residence is based on United Nations guidelines (.pdf, 2.4 MB); in the United Kingdom the population statistics are benchmarked to the results of the 2021 Census of Population.

Final results for the current population estimates can be found on the NISRA Mid-Year Population Estimates webpage.

2. Population Estimates for Northern Ireland

At the Northern Ireland level, population estimates are updated each year using the cohort component method.

2.1 Components of Change Method

In simple terms the previous year’s population estimate is “aged on” by one year, with births added and deaths removed. Net migration is also accounted for. The following formula is thus applied to update the population:

    Previous year’s population estimate aged on

    + Births to mothers resident in Northern Ireland;

    - Deaths;

    + Net migration (including movement of armed forces personnel)

The following sections describe how the different data sources are used in this calculation.

2.1.1 Census of population

The 2021 Census of population was the basis for the 2021 mid-year estimates.

2.1.2 Ageing of the population

Each year the population by single year of age is aged on by one year (for example, all two-year-olds become three-year-olds one year later).

2.1.3 Births

Birth figures by sex are obtained from registrations with the General Register Office. They include all births that occurred between 1 July and 30 June the following year, registered up to 31 December the following year. Births to non-resident mothers are not included, as evidence from Great Britain and the Republic of Ireland suggests that there is an imbalance between the number of non-Northern Irish mothers giving birth in Northern Ireland on one side, and Northern Ireland mothers giving birth outside Northern Ireland on the other.

2.1.4 Deaths

The General Register Office provides details of registered deaths by sex and single year of age at mid-year. It includes deaths that occurred between the 1 July and 30 June the following year, registered up to 31 December the following year. Also included are a small number of deaths that occurred prior to 1 July but were registered in the following year. Deaths of non-residents are included, under the assumption that equal number of Northern Ireland residents die abroad as non-residents die in Northern Ireland.

2.1.5 Migration into Northern Ireland

Migration into Northern Ireland or inflows is defined as persons who come to live in Northern Ireland from the rest of the United Kingdom or any other country for a period of at least one year. Inflows are estimated from the Medical Card Register, which is a list of patients registered with a family doctor. From this, the number, sex and age at mid-year of people who joined or rejoined the register within a one-year period is determined (this will include people who previously resided here but left and then returned). It is recognised that the Medical Card Register is deficient in recording young adult males; therefore, the age distribution of young adult males is adjusted to be similar to the young adult female age distribution.

2.1.6 Migration out of Northern Ireland

Migration out of Northern Ireland or outflows is defined as persons who leave Northern Ireland to live in the rest of the UK or any other country for a period of at least one year. Outflows are estimated from transferred and de-registered medical cards for flows to Great Britain and countries outside the UK respectively. These estimates are available by sex and age at mid-year. Previous research has shown that not all outflows are measured fully by the Medical Card Register and therefore de-registrations are scaled up by an additional 67 per cent (i.e. the assumption is that only 3 out of 5 persons who leave Northern Ireland to go abroad de-register from their family GP). This scaling factor was determined through analysis of historic population change using results from Censuses of population. As with inflows it is recognised that the administrative data is deficient in recording young adult males, therefore the age distribution of young adult males is adjusted to be similar to the young adult female age distribution. Migration flows to Great Britain are constrained to figures agreed by NISRA, the Office for National Statistics and National Records Scotland.

2.1.7 Armed Forces

Armed Forces are treated as a special population since i) they are not commonly on the Medical Card Register and ii) their age structure remains fairly stable each year. Due to periods of service, individual members of the forces move relatively frequently into and out of the coverage of population estimates. The adjustment procedure is to subtract Armed Forces based in Northern Ireland in the previous year’s estimates before ageing on the (civilian) population and then adding the Armed Forces back in after the ageing on is complete. The Defence Analytical Services Agency of the Ministry of Defence provides details on the number of forces stationed in Northern Ireland by age and sex on a quarterly basis, which can be found on the Defence and Armed Forces section of the gov.uk website.

3. Population Estimates for Areas within Northern Ireland

Population estimates for areas within Northern Ireland have been created using the Small Area Population Estimates (SAPE) note 1 methodology. The SAPE methodology uses a combination of the Components of Change and Ratio Change methods. Estimates resulting from the combination of these methods gives rise to a resultant population change which cannot be perfectly explained by the components of births, deaths and migration. As such, the remaining or unexplained difference is included in a category called “Other Changes”.

3.1 Components of Change method

For smaller areas within Northern Ireland, the Components of Change method is similar to that used for Northern Ireland as a whole. As before, the population is aged on by one year, births are added, deaths are removed, and an adjustment is made for migration. The data sources allow for this at the smaller area level due to the data being available with geographical indicators to georeference the place of usual residence.

At the Northern Ireland level, changes due to migration only involved movements to and from Northern Ireland. When estimating migration for areas within Northern Ireland, consideration must be taken of an additional component of migration, namely migration between areas within Northern Ireland (i.e. internal migration).

3.1.1 Migration within Northern Ireland

Internal (or within Northern Ireland) migration is defined as people moving their place of usual residence within Northern Ireland. Internal migration does not have an impact on the Northern Ireland level annual population estimates, only the distribution of the population across areas within Northern Ireland. To estimate internal migration between one year and the next, changes in the Medical Card Register are used. In addition, data from the Higher Education Statistics Agency (HESA) is used to account for under recording of migration in the Medical Card Register of students who move from the parental home to their place of study. Both datasets are available by age, sex and local area of origin and destination.

3.2 Ratio Change method

In the Ratio Change method, a selected indicator of population change is used to update the population from some earlier or base period. The method assumes an unchanged relationship over time between the chosen indicator and the true population . For example, if the chosen indicator increases by five per cent over the period for which estimates are required, the base population is also increased by five per cent. Further detail of this method is available in the original report on Small Area Population Estimates.

In the past a number of administrative data sources were selected as indicators of population changes within the ratio change method. These were selected on the basis of a high level of coverage of the population and acceptable quality. The weight given to each data source depended on its quality in terms of its historic coverage of the population and the accuracy of information on age, sex and geographical area. These sources are discussed briefly in the 2020 Mid-Year Estimates bulletin.

Changes to the availability of administrative data for use in creating population estimates for small geographical areas, including availability of and changes to Child Benefit data, necessitated a review of the Ratio Change methodology for this set of outputs. As a consequence of this review NISRA has maintained the methods utilised in the creation of the small area population estimates in the past (described in the 2020 Mid-Year Population Estimates methodology paper, but has changed the source data used to generate a ratio.

To ensure adequate coverage of the population and maintain a high standard of product, a single Statistical Population Dataset (SPD), developed by NISRA over a number of years, was used in place of a set of multiple complementary administrative datasets. For more details on the administrative sources that comprise this dataset, and the methodology adopted to create it, please see the Comparison of Northern Ireland Statistical Population Dataset with Census 2021, but in short it comprises data from the following sources:

  • The Business Services Organisation (BSO) provide information from the Medical Card Register, which is a comprehensive source of demographic information for NI. This administrative dataset includes an activity variable to indicate whether a person has had a recent interaction with primary healthcare services such as dental, ophthalmic or prescription services, or registered or changed address with their General Practitioner (GP).

  • Benefit claimant data from the Department for Communities (DfC) is used in a similar way to the medical card data, to include people who had recent activity.

  • Births registrations data from the General Register Office (GRO) for Northern Ireland provide data on births which can be used to count babies born in Northern Ireland in the previous six months. The aim is to count babies who may not yet have been registered with a GP on the medical card register. The individual record level data linkage reduces the chance of babies being double-counted.

  • Vital events data from GRO, such as both partners from marriage registrations, and parents from births registrations, is used as indicators of recent activity. GRO death registrations are used to remove people who died before the reference date. This information may also have been captured in the medical card register data.

  • The Electoral Office for Northern Ireland (EONI) provided the Northern Ireland Electoral Register and monthly updates which are used to provide indicators of activity. If a person was added to the electoral register or contacted EONI to change their details, that interaction was considered a sign of activity. The dates used were determined by the availability of full electoral registers.

  • The Department for the Economy (DfE) provide further education (FE) student enrolments and Training for Success (TfS), Apprenticeships Northern Ireland and Skills for Life and Work programme participants that are used to indicate recent activity.

  • Higher education student enrolments from the Higher Education Statistics Agency (HESA) are used to indicate recent activity and are also used to move Northern Ireland students to their term-time addresses. These datasets may have contained more recent activity data for young people who often are not interacting with the services covered by the other administrative datasets listed above.

Aside from having excellent demographic and geographic coverage across Northern Ireland, the SPD also has the facility of being available at a chosen reference date, allowing a degree of synchronisation with mid-year estimates on 30 June each year. The same rules for its creation can also be applied at different points through time, which is useful in Ratio Change, where comparison of population counts between years is a requirement.

Notes

  1. 2014 Mid-Year Population Estimates for Small Areas Statistical Bulletin

4. Geography

All data sources required for the Components of Change and Ratio Change methods are supplied with geographical indicators attached, allowing population estimates for areas within Northern Ireland to be created firstly by Super Data Zones (SDZs) note 2. These SDZs become the building blocks to create population estimates for most other geographies.

Notes

  1. Details on Super Data Zones can be found on the NISRA website

5. Average of Methods

The population estimates by SDZ sex and 5-year age band are derived from the average of figures created using the Components of Change and Ratio Change methods. These estimates are then subject to a process of quality assurance. In rare occasions, it is found that estimates from one or the other method appear to be outliers. The general solution to these cases is to rely solely on one method rather than the average.

Once the quality assurance at the SDZ level is complete, further work is undertaken to disaggregate these estimates by single year of age to allow aggregation to larger geographies such as Local Government District and Parliamentary Constituencies, and also to apportion SDZ estimates down to Data Zones note 3.

Both sets of disaggregations are achieved by apportioning the age-banded SDZ level estimates using a combination of (a) the Components of Change estimates (which are available at the lowest geographic level and by single year of age), and (b) the Statistical Population Dataset used in the ratio change method.

Notes

  1. Details on Data Zones can be found on the NISRA website

6. Data quality

Population and Migration Estimates for Northern Ireland are based on data gathered from statistical censuses and surveys, and data extracted from administrative or management systems. A brief outline of the main sources used to develop population estimates is given below. A more comprehensive outline of these sources, including details of the quality management actions undertaken to ensure that the data is suitable for population estimates, is detailed within the Administrative Data Quality Document.

Census

For information on the quality of Census data see the 2021 Census Quality Assurance section of the NISRA website.

Births and Deaths

Information supplied at birth / death registration is generally believed to be correct since wilfully supplying false information may render the informant liable to prosecution for perjury. Birth and death figures by sex (and also by single year of age for deaths) are obtained from registrations with the General Register Office (GRO) and all that occurred between the 1 July and 30 June the following year are included in the mid-year population estimates.

Migration

Migration is the most difficult component of population change to measure, as unlike births and deaths, there is no complete system for registering migration. Migration is estimated using transfers observed in medical cards note 4, detailing the list of patients registered with a family doctor:

  • inflows (persons who come to live in Northern Ireland for a period of at least one year) are estimated by counting the number of people who registered or re-registered with a family doctor; and

  • outflows (persons who leave Northern Ireland for a period of at least one year) are estimated by counting the number of people who de-registered with a family doctor.

When the medical card data are processed to calculate migration estimates, figures for migration to / from Great Britain are agreed between the different UK administrations to provide as much accuracy and comparison between UK administrations as possible for users.

Notes

  1. In previous reports, medical card data was referred as “health card” data.

7. Estimates of the population aged 90 and over

To produce single year of age estimates of the population aged 90 and over, NISRA has adopted the Kannisto-Thatcher Survivor Ratio Method note 5, an internationally recognised method used to provide a more detailed breakdown of the older population by age. Using death registration data, an estimate is produced of the number of people at a given age alive in a particular year. For the most recent year, the Kannisto-Thatcher Survivor Ratio Method uses an average of the last five years death data to produce an estimate of the number of survivors. For earlier years, if someone died aged 100 in 2024, this means that they were alive in 2023 aged 99, and aged 98 in 2022 and so on. This is used to produce age distribution profiles. The number of people aged 99 alive in 2021 is recalibrated from the estimated number of people alive aged 100 in 2024, plus the number of registered deaths of people aged 100 in 2024. One outcome of this method is that each year the estimates for earlier years become more accurate as more death data become available to inform age profiles. It also assumes that migration for those aged 85 and over is negligible. Estimates are then controlled to agree with the NISRA mid-year population estimates for those aged 90 and over.

Notes

  1. The Survivor Ratio Method for Estimating Numbers at High Ages, Thatcher R, Kannisto V, Andreev K, 2002. The Demography of Centenarians in England and Wales, Population Trends 96 pp5-12, Thatcher R, 1999.

8. Quality Assurance

Changes over time and annual population estimates are compared to several administrative data sources. These include those used in the estimation process, but can also include the active medical cards, the electoral roll, benefit claimants, and the number of domestic properties. Any significant differences found are examined further.

The improvements made are carried forward in population outputs going forward, with continued monitoring of the quality and availability of data sources and methodology. A close collaboration with colleagues in the Office for National Statistics and National Records of Scotland ensures continued improvement and comparability.

NISRA, September 2025

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