Loss and representativeness in a 53 year follow up of a national birth cohort (The 1946 Birth cohort)

Dr Gita MishraMRC National Survey of Health and Development

Department of Epidemiology and Public Health

Royal Free Hospital, University College Medical School, London

www.nshd.mrc.ac.uk

Acknowledgments

MRC NSHD Team membersProfessor Michael Wadsworth (Study director)Professor Diana KuhDr Marcus RichardsDr Rebecca HardySuzie ButterworthStephanie BlackRachel CooperImran ShahWarren Hilder

Benefits of the longitudinal or life course design

• Known sequence and chronology of development and ageing, and of exposures

• Near contemporaneous data collections– only short period of recall

• Ability to describe the diversity of underlying pathways to later health outcomes

The costs of the design in a long-running study

• The fixed sample structure

• Each data collection is informed by contemporary scientific ideas and methods

• Risk over time of increased sample loss and the consequent potential for bias

MRC National Survey of Health and Development (MRC NSHD)

AimsOriginally the study aimed to address 2 specific issues in the years before the establishment of the NHS– Reasons for falling fertility– Effectiveness of obstetrics and midwifery on

premature births, infant mortality, and on promotion of health of mother’s and infants

Has evolved into a life course study investigatingpathways to physical and cognitive ageing

MRC National Survey of Health & Development1946 birth cohort

 

Birth Registrations3rd - 9th March 1946 (N=16,695)

Population of the maternity survey (N=13,687)

 

Selection of follow-up sample of all single born, legitimatechildren of fathers in non-manual and agricultural employment

and 1 in 4 of all other single born legitimate children (N=5,362)   

 

 

Study of cohort first born offspring at ages 4 years and 8 years (N=2,205)

Study of women’s health, annually, at ages 47-54 years (N= 1,572)

MRC National Survey of Health & Development1946 birth cohort

 

Birth Registrations1 week 1946 (N=16,695)

Population of the maternity survey 1 data collection at birth (N=13,687)

 

Selection of follow-up sample of all single born, married womenwith husbands in non-manual and agricultural employment

and 1 in 4 of all other comparable births (N=5,362) 20 data collections from age 2 to 53 years

  

 

Study of cohort first born offspring at ages 4 years and 8 years (N=2,205)

Study of women’s health, annually, at ages 47-54 years (N= 1,572)

8 data collections

Maternity study of all births in 1 week in England, Wales & Scotland in 1946

Follow-up of a class-stratified sample (N=5362)

In infancy at 2 & 4 years

During school years at 6, 7, 8, 9, 10, 11, 13, 15 years

In early adulthood at 19, 20, 22, 23, 25, 26, 31 years

In middle adulthood at 36, 43, 53 years

Years Cohort ages National policy problems

1946 Birth Costs of maternity, reason forfalling fertility

1947-1950 1-4 years Social class differences in maternaland child mortality and morbidity.Value of health visitors’ work.

1951-1961 6-15 years Increasing the national level ofeducational attainment. The ‘waste of talent’ problem.

1962-1976 16-30 years Outcomes of education in terms ofoccupational choice and skills.Delinquency.

1976- 30 years onwards Ageing processes, self care ofhealth, receptivity to healthpromotion.

Maintaining the study’s momentum

Sources of information on sample loss

By age 53 years

• Unavoidable losses – Death (n=469 8.7%)– Emigration (n=461 8.6%)– Living abroad (n=119 2.2%)

• Avoidable or potentially avoidable– Permanent refusal (n=640 12.4%) *only 28 new cases

– Temporary refusal for this data collection only (first

classified at age 43 y) (n=280 5.2%)– Failure to trace (n= 330 6.1%)

Data collections & contacts with the sample (n=5362)

Year Age in Years

Respond-ent

Data Collector

Target Sample

% achieve

1946 -50 0-4 Mother HV 4993 95%

1951-61 5-15 Mother & child

SN or SD or HV & T

4900 89%

1962-77 16-31 All CMs P, HV, I 4858 78%

1965-71 19-25 Mothers of first born

RN 1783 94%

1982 36 All CMs RN 4838 86%

1989 43 All CMs RN 4826 87%

1993-2000 47-54 Women P 84-90%

1999 53 All CMs RN 3673 3035 (83%)

CM cohort member, HV health visitor, SN school nurse, SD school doctors, T teacher, I interview, RN research nurse, P postal

Attrition

The greatest overall attrition occurred in early adult years (16-31 years)– Cohort member could, for the first time, choose

whether to respond

– 5 out of the 7 data collections were by postal questionnaire

– Name and address changes were particularly frequent

– There may have been an adverse effect on response due to blurring of focus of the study aims during this period

Attrition con’t

The 3 later data collections (at ages 36, 43, and 53 years) have focussed strongly on health and obtained higher response rates than those earlier years

– Clear re-focus on health

– The employment of research nurses to collect data

– Introduction of summary feed-back of findings with a birthday card

– A clear explanation of the study’s aims in letters requesting each data collection

Birthday cards

– Birthday cards were introduced at age 16 years to encourage response after leaving school, which requests notification of changes of name and/or address

– They have been continued ever since but now include details of recent work, with references to recent publications

60th Birthday

Sample characteristics of avoidable losses (refusals or failure to trace)

Raised risk on avoidable loss were found in key variables

Childhood1. Shortness at age 4 years2. Experience of serious illness3. Late achievement of bladder control 4. Childhood social class of crowding5. Paternal manual social class, low cognitive test scores6. Low parental interest in education 7. Teachers’ ratings during adolescence of frequent problems

with discipline, disobedience and aggressionAdulthood

1. Adult social circumstances of low educational attainment2. Manual social class employment, 3. Not owning the home at 26 years 4. Not belonging to clubs or association 5. Being obese at 36 years was also associated with

avoidable loss from the study at age 53 years

Attrition from avoidable causes by quartiles of educational and cognitive score at 8 years

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

Year

Pro

po

rtio

n r

em

ain

ing

in

stu

dy

Low (Q1)

Quartile 2

Quartile 3

High (Q4)

Missing data and multiple imputation

• The importance of checking completeness is strongly emphasised at the nurse training sessions

• 73 % of those who provided data at 53 years, were also successfully contacted on 17 or more of the 20 data collections

• Only 7% of them had taken part in 10 or fewer previous data collections

• Multiple imputation is now used in analysis together with sensitivity analysis to deal with missing items/contacts– Growth and breast cancer risks– Diaries of alcohol consumption

Representativeness

• Representation is important not only for extrapolation, but also for estimating true prevalence, and for maintaining policy relevance

• There are some limitations on the representativeness of this sample– Selection predated the major immigration flows– Excludes births out of wedlock– Excludes multiple births

• Nevertheless it remains representative in most respects of the native population born in the early post war years

Sample Representativeness: Comparison of the weighted sample at age 53 years with 1991(50-54 yrs) census data

Full tim e em ploym ent

0

20

40

60

80

100

Males Females

Per

cen

tag

e

Social class

0

20

40

60

80

100

Non manual Manual Non manual Manual

Per

cen

tag

e

Highest educational qualifications - degree or higher

0

20

40

60

80

100

Males Females

Perc

enta

ge

Gender

0

20

40

60

80

100

Males Females

Per

cen

tag

e Census50-54 yrs NSHD NSHD

CensusAges 45 and up

Census16+ yrs

Sample Representativeness: Comparison of the weighted sample at age 53 years with 1991(50-54%) census data

Marital Status

0

10

20

30

40

50

60

70

80

90

100

Single Married Widow ed Separated/ Divorced

Single Married Widow ed Separated/ Divorced

Per

cen

tag

e

Males Females

Conclusions I

• A high rate of contact can be maintained

• Data collection with direct contact, such as home visits by a research nurse

• Provide information about the work of the study to the study members

– Introduction of summary feed-back of findings with the birthday card and website

– A clear explanation of the study’s aims in a letter requesting each data collection

Conclusions II

The responding sample continues in most respects to be representative of the national population of a similar age

Consistency of response over the study’s 20 data collections has been high.

The size of the sample responding in adulthood is adequate for the study of the major costly diseases and for the study of functional ageing and its precursors.

Although the problems inherent in the prospective design are unavoidable they are not, in the study described, a barrier to scientific and policy value

References

Wadsworth MEJ et al. JECH 1992; 46:3000-304Wadsworth MEJ et al. Soc Sci Med 2003; 57:2193-205Wadsworth MEJ et al. IJE 2006; 35:49-54Longford NT et al. JRSSA 2000; 163:381-402De Stavola et al. AJE 2004; 159:671-682

www.nshd.mrc.ac.uk