The Northern Ireland Longitudinal Study (NILS) has been formally available to researchers since the end of 2006. It comprises two major data linkage studies: the NILS and the Northern Ireland Mortality Study (NIMS). These were developed by the Northern Ireland Statistics and Research Agency (NISRA) and are maintained and managed by them as a resource for research. The availability of data on this scale is a new departure in Northern Ireland and it is hoped they will become established as part of the normal range of tools available to the research community. 

What is the NILS?

The Northern Ireland Longitudinal Study (NILS) is a large-scale, representative data-linkage study created by linking data from the Northern Ireland Health Card Registration system to the 2001 Census returns and to administrative data from other sources.  These include vital events registered with the General Register Office for Northern Ireland (such as births, deaths and marriages) and the Health Card registration system migration events data. Selection into the study is based on birth date (day and month): 104 dates throughout the year were selected and if an individual’s date of birth coincided with one of these they were included in the sample. The sample is large - c. 28% of the Northern Ireland population (approximately 500,000 individuals). It is designed for statistics and research purposes only and is managed by NISRA under Census legislation. The data are entirely anonymised at the point of use; access is only from within a strictly controlled ‘secure setting’ and governed by protocols and procedures to ensure data confidentiality.

Two similar record linkage studies are also available in Great Britain: the Office for National Statistics Longitudinal Study (ONS-LS) is a representative 1% sample of the population of England and Wales (approximately 500,000 people), while the Scottish Longitudinal Study (SLS) is a representative sample of 5.3% of the Scottish population (around 274,000 people). The ONS-LS, incorporating data from all Censuses since 1971 has been successfully running for over 25 years, while the SLS, currently incorporating linkage of the 1991 and 2001 Censuses, has been available for about three.  

What is the Northern Ireland Mortality Study (NIMS)?

The Northern Ireland Mortality Study (NIMS) is a large-scale data linkage study that links the 2001 Census returns for the whole of the enumerated population (approximately 1.6 million individuals) to subsequently registered mortality data from the General Register Office (GRO). While larger than NILS it is more limited in scope, focusing only on the linkage of mortality data. It allows researchers to focus on more detailed analyses of specific cause of death, some of which may not be possible in NILS because of small numbers in sub-populations and the analysis of less common causes of death (e.g. accidental death). The NIMS dataset is recommended to researchers whose primary interest is in mortality in Northern Ireland. These data are maintained under the same conditions as the NILS and is accessible only under the same constraints (see above).

What can the NILS be used for?

The NILS is best seen as a dynamically evolving data resource based on an accumulation of Census and other routinely collected vital events and contextual data required by government. It is representative of the whole population and is held on a continuing sample of around 28% of the Northern Ireland population. At its Core is the census information – including demographic, socio-economic, self-reported health, housing, household and family structure data, all of which are available for analysis. For each individual included in the sample the Census information recorded for other members of the household is also available, allowing the detail of the household context to be used. These data, however, are not linked with the vital events data.  Also included are related (but non-census based events such as migration history and deaths and births) occurring to NILS sample members. This is supplemented by area-level (or contextual) data which can be also be included for analysis. Full information about the data and linkages can be viewed in the Data section and more detail on NILS variables can be found in the Data Dictionary.

For examples of the range of possible research opportunities available please refer to the links to all three Longitudinal Studies where regularly updated lists of published papers based on current and previous projects are available.  Both the NILS dataset itself and access mechanisms are modelled on the ONS-LS. The ONS-LS, SLS and now the NILS offer the potential to look at a wide range of important social and epidemiological issues including occupational mortality; fertility change; family structure; women’s occupations; geographical and social inequalities in health; social mobility; and the health of ethnic minorities and religious groups. This is evidenced in numerous academic and policy outputs, much of which has fed into government social and health policy initiatives.

Importantly for local research capacity, NILS datasets can be used in analyses of social inequalities in health – an area of enquiry which should assume greater importance when the 2011 Census data are linked, allowing, for the first time, analyses of socio-economic and socio-demographic transitions in the decade between the 2001 and 2011 Censuses.      

Data of this type are flexible and can be used in a wide range of analyses: from descriptive tabulations; logistic and linear regression; survival analysis using Cox proportional hazards modelling; and multilevel modelling. The preferred analysis tools for researchers are Stata, SPSS and MLWin and available statistical techniques are limited only by these research tools.

How are the NILS data managed?

NISRA provides ´safe data in a safe setting´ and maintaining this is the paramount concern of all NILS staff. The data are confidential in nature and not accessible outside the safe setting. The main databases are constructed as separate repositories held permanently on an isolated computer intra-net and accessible only to NISRA staff administrators. Researchers wishing to use the data should contact NILS-RSU who provide controlled access to data sub-sets specified for individual projects. They will also help researchers formulate research questions, submit applications to the Research Access Group and assist with analysis and interpretation. For further information see Access.

Is the NILS for me?

The NILS is designed to allow longitudinal analysis utilising both Census and linked vital events data and other data collected for routine administrative purposes. It also has the capacity to link area level contextual data from sources outside NISRA. Of especial interest is the capacity to link disparate individual level health services data for specially defined one-off studies – though because of the sensitivity of such studies they are subject to increased scrutiny and heightened ethical and privacy protection protocols. Also of special interest is the future linkage of 2011 Census data which will allow examination of intercensal socio-economic and demographic transitions.

While the NILS has been funded specifically to provide a resource for health-related research for both the academic and government sectors the definition of health is interpreted in terms of the World Health Organisation statement – that is, very liberally and in terms of overall well-being.

For more information on whether or not the NILS can contribute to your research contact the NILS Research Support Unit.