A variety of administrative and statistical data are linked in the NILS. Most of the data come from the 2001 Census, General Register Office (GRO) vital events and demographic data derived from health registrations. Full details are available in the Data Dictionary. A Cohort Profile of the data has been published in the International Journal of Epidemiology.

Selection into the study is based on annual birth date (day and month): 104 dates throughout the calendar year were selected and inclusion triggered if an individual’s date of birth coincides with one of these. These sample ‘members’ form the spine of the study and all data.included relates to this core group. For each member additional information is collected for all those living in the same household.  Vital events data and migration events are linked for sample members only. Indicators of migration events are taken from regular six monthly downloads of health registration data in which change of address information is recorded.

Available datasets include:

2001 Census data

Age, sex and marital status
Family, household or communal establishment type.
Housing, including tenure, rooms and amenities,
Country of birth, ethnicity, religion
Educational qualifications
Economic activity, occupation and social class
Migration
Long-standing illness, self-rated health, care giving

This data is available for both NILS members and for all members of households with at least one NILS member.

Vital Events (GRO Data)
 
New births into the sample
Births to sample mothers and fathers
Stillbirths to sample mothers and fathers
Infant mortality of children of sample mothers and fathers
Deaths of sample members
Migration Events (Health Card Data)
Immigrants into the sample
Emigration out of Northern Ireland of sample members
Re-entries into Northern Ireland after previous emigrations of sample members
Migration within Northern Ireland of sample members

VLA data
 
The VLA are responsible for rating assessments in Northern Ireland and hold a database of all properties with associated household characteristics such as number of rooms, property type, floor space, central heating etc.  This information is linked to the households of the NILS members.  Also included are the capital values of each property. These are taken from the recent Northern Ireland Rating Reform which used house values as at 1 January 2005 to calculate new rate charges.