NATIONAL CHILD DEVELOPMENT STUDY

The National Child Development Study (NCDS) is a continuing, multi-disciplinary longitudinal study which takes as its subjects all the people born in one week in England, Scotland and Wales in one week in March 1958. 

NCDS has its origins in the Perinatal Mortality Survey. Sponsored by the National Birthday Trust Fund, this was designed to examine the social and obstetric factors associated with stillbirth and death in early infancy among the children born in Great Britain in that one week. Information was gathered from almost 17,500 babies. NCDS was the second in a series of four such perinatal studies, the others being based on a week's births in 1946 and 1970, and on births in selected wards in 2000/01. Each has formed the basis of a continuing longitudinal study.

As the table below shows, following the initial birth survey in 1958, there have to date been eight attempts to trace all members of the birth cohort in order to monitor their physical, educational, social and economic development. These were carried out by the National Children's Bureau in 1965, 1969, 1974, and 1981; by the Social Statistics Research Unit, City University, in 1991; and by the Centre for Longitudinal Studies, IoE in 1999/2000, 2004 and 2008.

It is important to note that the birth cohort was augmented by including immigrants born in the relevant week in the target sample for the first three follow-ups (NCDS 1-3). This has implications for both the target and achieved samples in the table below, which indicates the target and achieved samples for each follow-up. These figures were revised between NCDS6 and NCDS7 in the light of an exercise conducted within CLS to examine the changes in the NCDS and BCS70 populations and samples over time.  This is available as a CLS Technical Report.

Notes:

1. Cross-sectional target sample - immigrants with appropriate date of birth included for NCDS1-3.
2. Longitudinal target sample - only those born in Britain and still alive/not permanently emigrated.
3. Cross-sectional achieved sample - immigrants with appropriate date of birth included for NCDS1-3.
4. Longitudinal achieved sample - only those born in Britain and still alive/not permanently emigrated.
5. For a random sample of one in three cohort members.  Information was collected directly from 3,008 children of cohort members.
6. This could be the Cohort Member, their Spouse, or Partner (same 1 in 3 random sample).  2,588 mothers completed the 'mother' questionnaire, giving information on 4,278 children.

* Figures for target samples for NCDS7 and NCDS8 are not currently available, as there is a project under way to rationalise and consolidate information on the exact numbers of deaths in NCDS.  Figures should be available shortly.

There are two ways in which the target and achieved samples can be conceptualised:

1. the longitudinal target sample consists of all those born (alive or dead) in Great Britain in that particular week in March 1958, 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 (at least one survey instrument partially completed).
2. the cross-sectional target sample at a particular sweep consists of all those born anywhere in the world in that particular week in March 1958, and living in Britain at that sweep.   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).

After the birth survey, anyone born abroad during that week who subsequently moved to Britain before the age of 16, may be included in the cross-sectional sample, but not the longitudinal one.  Immigrants were added for the sweeps NCDS1-3, but no subsequent attempts were made to include further members.

For a rigorous analysis of response rates, refer to the Technical Report.

In addition to the seven attempts to trace all members, contact was made in 1978 with the schools attended by members of the birth cohort at the time of the second follow-up in 1974 in order to obtain details of public examination entry and performance. Similar details were also sought from sixth-form colleges and further education colleges, etc where these were identified by schools.

For the birth survey, information was obtained from the mother and from medical records by the midwife. For the purposes of the first three NCDS surveys, information was obtained from parents (who were interviewed by health visitors), head teachers and class teachers (who completed questionnaires), the schools health service (who carried out medical examinations) and the subjects themselves (who completed tests of ability and, latterly, questionnaires).

The 1981 and later surveys differ, in that information was gathered by professional survey research interviewers.  In 1981 information was obtained from cohort members and from the 1971 and 1981 Censuses - from which variables describing area of residence were taken.  In the 1991 survey there was a professional interview with the cohort member, but self-completion questionnaires were also used to gather data from NCDS subjects and from husbands, wives, and cohabitees. In addition, for a random sample of one in three cohort members, information was collected for all natural or adopted children who were living with them. Data were gathered from the children themselves, and from their mother, or mother-figure (who might be the cohort member, or their spouse or partner), using a series of age-specific assessments of cognitive and behavioural development. These were supplemented by a mother interview, and by interviewer observations of mother-child interaction.  For the 1999-2000 sweep, information was obtained from cohort members by interviewer and self-completion, but using CAPI (Computer-Assisted Personal Interviewing) for the first time.  The 2004 survey was administered by telephone.

During the collection of exam data in 1978 information was obtained only from the schools and colleges by postal survey.

Note on representative nature of immigrants within sample

A question is sometimes raised about children who were born abroad in the specified week and were later included - are they a truly representative sample of immigrants?

In organising the NCDS1-3 follow-ups (ages 7, 11 and 16 years) contact was not maintained with study families/individuals between surveys.  The survey organisers sought to identify the target sample for each follow-up by asking all private or local authority-run schools in Great Britain to provide details of all pupils with an appropriate birth date.  In this way they hoped to include in the target sample all those living in GB with an appropriate birth date, irrespective of whether they had been included in an earlier follow-up.  They also made use of NHS records and advertising/media appeals to seek to find those included in earlier surveys that could not be found among the school lists. 

Once individuals had been identified, they contacted parents/parent-figures to solicit their involvement in the specific follow-up.  Data was subsequently gathered from parents by health visitors and from schools; and from the subject children with the assistance of teachers and the doctors and nurses of the Schools Health Service.  Completed questionnaires and assessments were subsequently returned to the NCDS team (then based at the National Children’s Bureau in London) for checking and further processing.

Thus, the data generated by each follow-up reflects the outcome of major collaborative enterprise.  While there was scope for loss of sample members and/or data at many stages, there is little evidence to suggest that the follow-ups do not include the majority of children included in the target samples (see Technical Report).

It seems safe to assume this is true for the ‘immigrant sample’ also, although no research appears to have been done on this specific issue.  To the extent that the procedures adopted to identify, recruit and survey the target samples may be considered robust and appropriate, it seems unlikely that a large proportion of migrant children who may have been living in Britain at the time of the NCDS1-3 surveys were inadvertently excluded from follow-up. 

However, there is some suggestion that certain immigrant groups are less likely to participate in NCDS4 and later surveys (see, for example: Shepherd, P. (1993) Analysis of response bias. In Ferri, E (ed), Life at 33: the fifth follow-up of the National Child Development Study. London: National Children's Bureau, City University, Economic and Social Research Council).