About the sample
The NCDS follows the lives of all people born in England, Scotland and Wales in one particular week of March 1958. In the first three sweeps (at ages 7, 11 and 16), the target sample was augmented to include immigrants born in the same week.
The NCDS has both longitudinal and cross-sectional samples.
The longitudinal target sample consists of all those born (including stillbirths) in Great Britain in the particular week, until they die or permanently emigrate from Britain. The longitudinal achieved sample is all those members of the longitudinal target sample who participate in a particular sweep (meaning at least one survey instrument partially completed).
The cross-sectional target sample for each sweep consists of all those born anywhere in the world in the same week as the longitudinal sample, who are living in Britain at the time of the sweep, and who joined the NCDS sample during the school years. The cross-sectional achieved sample is all those members of the cross-sectional target sample who participate in a particular sweep (at least one survey instrument partially completed).
The cross-sectional sample only includes immigrants who moved to Britain before the age of 16, as there were no attempts to include further members beyond the age 16 survey in 1974.