BRITISH COHORT STUDY

The 1970 British Cohort Study (BCS70) is a continuing, multi-disciplinary longitudinal study which takes as its subjects all those living in England, Scotland and Wales who were born in one particular week in April 1970.

BCS70 began when data were collected about the births and families of just under 17,200 babies born in England, Scotland, Wales and Northern Ireland in a particular week in April, 1970.  At this time, the study was named the British Births Survey (BBS), and it was sponsored by the National Birthday Trust Fund in association with the Royal College of Obstetricians and Gynaecologists.  (Subjects from Northern Ireland, who had been included in the birth survey, were dropped from the study in all subsequent sweeps). 

Since 1970 there have been six attempts to gather information from the whole cohort, as the chart below shows. With each successive attempt, the scope of enquiry has broadened from a strictly medical focus at birth, to encompass physical and educational development at the age of five, physical, educational and social development at the ages of ten and sixteen, and then to include economic development and other wider factors at 26, 29 and 34 years.

Data have been collected from a number of different sources, and in a variety of ways. In the birth survey, information was collected by means of a questionnaire that was completed by the midwife present at the birth, and supplementary information was obtained from clinical records. The five-year and ten-year surveys were carried out by the Department of Child Health, Bristol University and the survey at these times was named the Child Health and Education Study (CHES) . In 1975 and 1980, parents of the cohort members were interviewed by Health Visitors, and information was gathered from head and class teachers (who completed questionnaires), the school health service (which carried out medical examinations on each child), and the subjects themselves (who undertook tests of ability). In both 1975 and 1980, the cohort was augmented by the addition of immigrants to Britain who were born in the target week in 1970.

The 1986 survey was carried out by the International Centre for Child Studies and named Youthscan. In this sweep, sixteen separate survey instruments were employed, including parental questionnaires, school class and head teacher questionnaires and medical examinations (including measurement of height, weight and head circumference). The cohort members completed questionnaires, kept two four-day diaries (one for nutrition and one for general activity), and undertook some educational assessments.

The 1996 follow-up was carried out by the Social Statistics Research Unit, City University. It was based on a postal survey of cohort members for whom a current address was available.

The most recent follow-ups in 1999/2000 and 2004 were managed by the Centre for Longitudinal Studies, and fieldwork was carried out by the National Centre for Social Research.

The table below indicates the target and achieved samples for each follow-up.  These figures were revised in the light of an exercise conducted within CLS between the 29-yr and 34-yr sweeps, to examine the changes in the NCDS and BCS70 populations and samples over time, which is available as a CLS Technical Report. 

British Cohort Study 1970: Target and Achieved Samples

Notes:

a: Cross-sectional target sample (emigration estimated) - immigrants with appropriate date of birth included.
b: Longitudinal target sample (emigration estimated) - only those born in Britain and still alive/not permanently emigrated.
c: Cross-sectional achieved sample - immigrants with appropriate date of birth included.
d: Longitudinal achieved sample - only those born in Britain and still alive/not permanently emigrated.

* Although one might expect a cross-sectional target sample to be a higher figure than the corresponding longitudinal target,  the reverse is true at age 5.  The explanation is that cross-sectional target figures exclude temporary emigrants, whereas longitudinal targets include them (see CLS Technical Report, Table 3.1).

** Child assessments were administered to a 1-in-2 sample.

*** Figures for target samples for the 2004 follow-up are not currently available, as there is a project under way to rationalise and consolidate information on the exact numbers of deaths in BCS70.  Figures should be available in due course.

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

(a) the longitudinal target sample consists of all those born (alive or dead) in Great Britain in that particular week in April 1970, 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).

(b) the cross-sectional target sample at a particular sweep consists of all those born anywhere in the world in that particular week in April 1970, 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, may be included in the cross-sectional sample, but not the longitudinal one.  Immigrants were added for the sweeps up to and including age 16, but no subsequent attempts were made to include further members.

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

For the 1999-2000 and 2004 sweeps, information was obtained from cohort members by interviewer and self-completion, but using CAPI (Computer-Assisted Personal Interviewing), unlike the earlier sweeps.

As the chart above shows, response rates varied between survey sweeps. The lower response at 16 arose because a teachers' strike at the same time resulted in many subjects not receiving their questionnaires. A later attempt was made to improve response rates by sending appropriate survey documents to the cohort member's homes. The 1996 follow-up targeted the four in five cohort members for whom a current address was available and, as a result, the number of respondents was lower.

In addition to the six major follow-ups, sample surveys were carried out in 1972/1973 and in 1977. In 1972/1973 the British Births Child Survey took as its subjects three sub-samples; all twins in the original cohort, the low-birth-weight and post-mature births, and a random ten per cent of the original cohort. The South-West Region Survey , also carried out at this time, included ninety five per cent of the cohort members who lived in the south west of England or Glamorgan, South Wales. These smaller scale surveys were undertaken so that the large gap in child development terms between birth and five years of age (when the next full sweep was planned) could be bridged. In 1977, an attempt was made to assess the effect of non-response to the five year survey when non-responders were traced and interviewed.

In 1992, a further sample survey of 1,650 cohort members was undertaken by the City University Social Statistics Research Unit (SSRU - the name by which the CLS was known before moving to the IoE). The major aims of the survey were to investigate the forces and circumstances which influence young people in their transition from full-time education to employment; and to examine the extent of literacy and numeracy problems in the cohort, what underlay them, and how they affected employment. In addition, this survey was designed to serve as a feasibility study for a full sweep of the cohort in their early twenties. For this sample, survey information was gathered by survey research interviewers.

Data available for research

Data from the birth surveys and all the subsequent five sweeps are now lodged at The Data Archive, University of Essex.

Access to the data is open to anyone interested, although intending users are asked to commit themselves to ensuring that confidentiality is observed, and to inform the Cohort Studies User Support Group at the Centre for Longitudinal Studies (CLS) about their proposed use of the data and any resulting publications.