Cohort description: the Northern Ireland Longitudinal Study (NILS).

Northern Ireland has traditionally been well served with cross-sectional studies, including episodic surveys of poverty and social exclusion, and of health and social well-being. However, such studies are of limited use in either analysis of life-course transitions or in the separation of cause and effect, both important goals of current research strategies where renewed interest in equity and social exclusion is stimulating research into the effects of disadvantage on individuals over time. This is all the more urgent given the increased social and geographical mobility and greater fluidity in people’s lives. Such information can only be derived longitudinally, and there was a dearth of such large-scale general purpose studies in Northern Ireland. Information had not been included in the three British Birth Cohort studies of 1946, 1958 and 1970 and although other longitudinal studies, such as PRIME and Young Hearts, are available they were designed to answer research questions related to specific diseases. Although the British Household Panel Study has recently been extended to include Northern Ireland and there is now a Northern Ireland component to the Millennium Birth Cohort Study, neither is large enough to function as a general purpose longitudinal study to meet the general research or policy needs of Northern Ireland. In the early 2000s, a group of senior statisticians from the Northern Ireland Statistics and Research Agency (NISRA) and academics was convened to estimate the cost of a Northern Ireland Longitudinal Study (NILS), equivalent to others either already available in England and Wales or, at the time, in development in Scotland. The aim was to have a multi-cohort study that would fulfil a range of academic and policy-related purposes, with a sample size large enough to enable robust analysis of population sub-groups and of areas of policy relevance. Because the experience of the Scottish Longitudinal Study (SLS) team had shown that linkage to the 1991 census would incur considerable costs, especially if social class for earlier years had to be recoded to the NS-SEC classification used in the 2001 census, it was decided to start with the 2001 census. Funding for the establishment and maintenance of NILS (and its sister study NIMS, see below) was jointly secured in 2003 from the Department of Health, Social Services and Public Safety and the Research and Development Office of the then Health and Personal Social Services. All funding for the development and maintenance of NILS and NIMS now comes from the Health and Social Care Research and Development Division of the Public Health Agency (HSC R&D Division). NISRA helps to fund the NILS/ NIMS project both through the provision of accommodation to house all aspects of the NILS/NIMS operation and staff to maintain and develop the databases and provide strategic management of the project. Both NILS and NIMS were launched in December 2006.