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What is a Cohort Study?
Jane Elliott
Director of the Centre for Longitudinal Studies
July 2012
Sub-brand to go here
CLS is an ESRC Resource Centre based at the Institute of Education
ObjectivesIntroduction to the content and design of the British Birth Cohort
studies at CLS
Further examples of cohort studies
The distinction between a cohort study and a panel study
Examples of recent research using cohort data
Examples of how cohort studies have had an impact on policy
How to find out more/access the data
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British Birth Cohort Studies
1946: MRC National Survey of Health & Development
1958: National Child Development Study
1970: 1970 British Birth Cohort Study
MCS: Millennium Cohort Study — first national birth cohort study for 30 years (2000-1)
Plans for a 2012/2013 cohort study
4
A Roman Cohort?
A cohort ....individuals moving or progressing as a unit
1958 Birth Cohort Study
Sample of over 17,000 infants born in 1958 (perinatal mortality study)
Sample followed at ages 7, 11, 16, 23, 33, 42, 46, 50 (biomedical at age 44)
Multipurpose study: family life; education; employment; skills; housing; health; finances; citizenship
Approximately 10,000 individuals are still participating
Mainly quantitative – highly structured interviews, but qualitative interviews with a subsample of cohort members at age 50
Core funded by ESRC with data collected every four years (five years after age 50)
PMS NCDS1 NCDS2 NCDS3 NCDS4 NCDS5 NCDS6 Biomedical NCDS7 NCDS8(1958) (1965) (1969) (1974) (1981) (1991) (2000) (2002-3) (2004-5) (2008-9)Birth 7 11 16 23 33 42 44-45 46 50
17,733a 16,883 16,835 16,915 16,457 15,600 15,145 12,037 11,739 12,316
Mother — Parents — Parents — Parents
School — School — School
Tests — Tests — Tests Tests
Medical — Medical — Medical — Medical
Subject — Subject — Subject — Subject — Subject — Subject — Subject — Subject — Subject
Census — Census
Spouse/ Partner Biometric measures
Consents to linkage
MothercBlood samples
Children Saliva sample
17,415b 15,425 15,337 14,647 12,537 11,407 11,419 9,349 9,534 9,793
Notesa: Target sample - Excludes emigrants, refusals & deaths. Includes immigrants at NCDS1-3.b: Achieved sample - At least on survey instrument partially completedc: Mother - Could be Cohort Member or spouse/partner
NCDS Follow-ups and sources of information 1958-2010Original sample: all living in GB born in one week in 1958
Biographical interview
Hypothetical life history
x
Born
1958
1st Child 1984 2nd Child
1987Age 7 Age 11 1991 2000
Age 42
2004
Age 46
Age 16
Age 23
1981
Age 33
Gets married
Parents’ social class
Parental interest in school work
Free school meals
Mother smoking
Parental divorce
Maths and reading tests
Teachers’ assessment of child’s behaviour
Exam results
Job 1 Job 2 Job 3
Voting behaviour
Psychological well being
Working hours preferences
Savings
Domestic division of labour
Union membership
Training and skills
Aspirations
Structured interview at age 50Household composition, marital status etc
Housing
Relationships & domestic division of labour
Children and parents
Family income
Employment and Partner’s employment
Pensions & attitudes to retirement
Qualifications, training and skills
Health (including menopause)
Alcohol consumption and smoking behaviour
Memory & Concentration
Voting behaviour, social participation & social support
Well-being
Consent to record linkage
Overview of the 1970 cohort study
• BCS70 is a multi-disciplinary longitudinal study following the lives of over 17,000 individuals born in Great Britain in one week in 1970
• There have so far been 8 sweeps of data collection: (Birth, 5, 10, 16, 26, 30, 34 and 38)
• The 9th sweep will take place in 2012 when study members will be aged 42: 75 minute face to face interview Additional self-completion questionnaire (paper or online)
• Funded by ESRC
Oversamples for Scotland, Wales and Northern Ireland, areas with high child poverty and, in England, areas with higher ethnic minority populations
Over 19,000 children born in the UK between September 2000 and January 2002
Follows children throughout their lives
Fifth survey, when children are in their last year of primary school
Funded by ESRC and UK government departments
One of five British Birth Cohort Studies
Overview of the Millennium Cohort Study
What have we done before? And what’s new?
Narrative elements of cohort studies
Allow us to trace lives through time & understand how childhood circumstances may impact on adult outcomes
Potentially allow for the construction of individual case studies based on detailed information collected over the years (while preserving confidentiality)
Allow for a focus on the historical context which has helped shape individual experiences
Comparisons between cohorts can enable the development of a narrative about social change
1958 and 1970 birth cohorts
0
10
20
30
40
50
60
1950 1960 1970 1980 1990 2000 2010 2020
Year
Ag
e
NCDS
BCS70
Cohort Comparisons
7
11
16
23
33
46
50
42
5
10
34
30
26
16
38
Life cycle effects
Source: Exploring Data (C. Marsh 1988) Figure 6.1 Unemployment as a problem in Britain: actual and perceivedSource: unemployed claimant count: Employment Gazette Decembet 1982 and May 1986. Percentage naming unemployment as most or second most urgent problem facing the country: Gallup Political Index monthly.
Proportion of women in paid employment, by age and cohort
Source: Jenny Neuburger - Paper presented at CLS June 2008
What is the difference between a cohort study and a (household) panel study?
Cohort Study1. All individuals in the sample are the
same age
2. Data collection focussed on specific life stage
3. Large sample of the same age facilitates modelling of specific life transitions
4. Focus on individual trajectories (limited info about other hhold members)
5. Data collection may be less frequent (e.g. British Birth Cohort Studies)
6. Objective health measures in childhood – resource for biomedical research
Panel Study1. Sample of adults representative of
whole age range
2. Questions need to be applicable to all
3. Sample spanning different ages allows for description of whole population
4. Focus on household dynamics and transfers
5. Data collection every year or perhaps every two years
6. Perhaps less likely to have a focus on health/bio-medical data
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Qualitative Resources
•Children’s Essays
•Biographical Interviews at age 50
•Open ended question at age 50... ‘Imagine you are 60’
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Social participation project: Qualitative Sub-study of NCDS 2008
• ESRC funded a qualitative component of the 2008 sweep of the 1958 cohort study
• Initial aim to interview 180 cohort members in 3 separate areas of Britain
• 170 interviews carried out in England and Scotland and a further 50 in Wales
• Aim to provide archived qualitative data for secondary analysis, together with the existing quantitative datasets
• Joint project with CRESC at Manchester: Jane Elliott; Andy Miles; Sam Parsons; Mike Savage
• Funded by the ESRC Research Resources Board
Structure of the interview and topic guide• Interview in six sections:
o Neighbourhood and belongingo Social participation and leisure activitieso Friendshipso Life story and trajectorieso Identitieso Experience of the NCDSo Visual exercises also included – personal community maps and life
trajectories
• Aim for an average of ninety minute interviews• Interviews digitally recorded and transcribed verbatim• Interviewers also wrote a brief summary of the interview• Interviews to be anonymised and deposited at the UK data archive
Age 50 self-completion questionnaire
At age 50, in 2008 NCDS Cohort members completed a 16-page paper self-completion questionnaire, including questions about health and well being and a personality inventory.
The final question stated:
Imagine that you are now 60 years old...please write a few lines about the life you are leading (your interests, your home life, your health and well-being and any work you may be doing).
Mean length of 7383 responses: 57 words
All of the 7383 responses have been transcribed and are being documented and prepared for deposit at the data archive.
Basic Skills – a major policy concern in Britain
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The value of basic skills in the British labour market: Vignoles, De Coulon and Marcenaro-Guierrez (Oxford Economic papers 2011) Aim to evaluate the labour market value of literacy and numeracy
Draw on literacy and numeracy tests carried out with the BCS70 cohort at age 34 in 2004.
Make use of test score information collected during childhood and also information on qualifications and employment history in order to isolate the impact of basic skills on wages
Cross cohort analysis carried out to assess whether the wage return to skills has changed over time.
Richness of data from the cohort studies allows proper control for a wide range of observable characteristics.
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24
25
26
27
The value of basic skills? Vignoles, De Coulon and Marcenaro-Guierrez (Oxford Economic papers 2011)
Models demonstrated that literacy and numeracy skills have a significant relationship with earnings even for individuals with similar levels of education
The effects appear to be very similar for both men and women
One standard deviation difference in skill levels is associated with approx 15% increase in earnings
Cross-cohort comparisons suggest that the value of basic skills has remained stable over time – this implies that the increase in supply of skills has been matched by an increase in demand for skilled workers
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Poverty, family resources and children's early educational attainment: the mediating role of parenting (Kiernan and Mensah, 2011, British Ed Research Journal)
• Uses longitudinal data from first three waves of MCS (N=5462)• Focus on both episodic and persistent poverty• Outcome measure is based on the Foundation Stage Profile collected from
teachers when children were aged 4-5 in the first year of primary school• Equivalised family income was used to measure poverty• Family resources index was also constructed: income poverty, mother’s
education, family employment, housing tenure, quality of the local area, mother’s age at birth of her first child, family structure, number of children in the household, child’s birth order, child’s ethnic origin and the language spoken in the home
• Parenting measure constructed using reports of activities with child and interviewer observation
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Poverty, family resources and children's early educational attainment (results summary)Children from poor families and those with low levels of family resources
are doing worse in their first year of school
Poverty matters but persistent poverty is even more detrimental
The parenting index was also a very important factor predicting children’s early educational attainment
Positive parenting matters regardless of the levels of resources in the family
A decompositional analysis suggested that about one-half of the effects of child poverty and 40% of resource disadvantage may be accounted for by the quality of parenting the child has received in early childhood
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How do I access the data?Data is available via the Economic and Social Data Service based at the
University of Essex
Data is free for non-commercial use (e.g. teaching and research)
Following a simple registration procedure data can be downloaded directly from the Economic and Social Data Service website
Information about what questions have been asked is provided via the CLS website but we can also answer individual enquiries about specific topics.
Plans to make more potentially disclosive data available via the Secure Data Service at the University of Essex:
http://securedata.data-archive.ac.uk/home
CLS Birth Cohort Studies: Web ResourcesResources available via CLS website:
(www.cls.ioe.ac.uk/Cohort/mainncds.htm)– Searchable bibliography (over 2000 articles, chapters etc)– Briefings (summaries of recent findings)– Annotated Questionnaires and CAPI Documentation– Technical Reports – e.g. on sampling, instrument development and
fieldwork of MCS– Data Dictionaries – Events and workshops
Policy relevance of 1958 and 1970 cohort studiesThe health impact of smoking in pregnancy
Child poverty
Declining intergenerational mobility
Antecedents and consequences of disability
Health continuities over the lifecourse and health inequalities
Determinants of crime and anti-social behaviour
Social and economic returns to education and training
Access and barriers to higher education
Improving adult basic skills
Women’s opportunities in employment
Maternal employment and child outcomes
Conclusions: research questions best addressed by birth cohort data
Long term outcomes of experiences and decisions in early life
Medium and short-term outcomes & links between different life domains (e.g. health and employment)
Descriptions of individual trajectories – careers, relationships, fertility, poverty and disadvantage
The links between social change and the changing experiences of different cohorts
Intergenerational transmission of advantage and disadvantage and the processes involved
Websitewww.cls.ioe.ac.uk
Please register for regular updates
Follow us on twitter at:
www.twitter.com/Clscohorts
Overview of the Cohort Resources Facility
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Leadership team
WP6: Data linkage - geography
WP5: Data linkage administrative data
WP7: Data linkage – health data
1946 cohort
1958 cohort
1970 cohort
ALSPAC
MCSUnderstanding
Society
SWS
HCS
New BCS
Cohorts’ CollegeCohorts’
repository
Metadata
Uniform Search Platform
Training and capacity building
Impact
BCS70 Sample size (Previous sweeps) - additional information
0 (1970)
5 (1975)
10 (1980)
16 (1986)
26 (1996)
30 (2000)
34 (2004)
38 (2008)
Total 16,571 13,071 14,874 11,621 9,003 11,621 9,665 8,874Postal Survey
Telephone Survey
Achieved sample size in previous sweeps
N % Eligible
Productive 8874 75.6
Non-contact 1949 16.6
Refusal 712 6.1
Other unproductive 198 1.7
Ineligible (Died / Emigrated) 110 -
Total Issued Sample 11,843
Age 38 Telephone Survey Response
A different cohort design – the Hertfordshire Cohort Study
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Hertfordshire Cohort Study: structure
Live births in Hertfordshire recorded in Health Visitor’s ledgers39,764
Traced and living in Hertfordshire8,650
Home interviews3,225
Clinical outcome questionnaire2,225
Dead215
Musculo-skeletal follow-up642
Clinic visits2,997
Hertfordshire Cohort Study: measurements
Historical records: Birth weight, weight at one yearInfant feeding/infections
Questionnaire: General healthHRQoL: SF36Diet: FFQ and 24 hr diaryPhysical activity
Clinic visit: CVD: blood pressure, ECGCOPD: spirometry
Type 2 DM: anthropometry, OGTT Osteoporosis: DXA
OA: Hand & knee x-raySarcopenia:strength, anthropometry,
pQCTFrailty: Fried score, Rockwood indexWellbeing: WEMWB Scale
Venous blood: Glucose, insulin, bone turnover, DNA
Syddall et al Int J Epidemiol 2005
Hertfordshire Cohort Study: geography
Hertford
Radlett
Royston
Welwyn Garden City
Southampton
Baldock
Hertfordshire Cohort Study: history