Transcript Life course epidemiology - Medical Research Council

Loss and representativeness in a 53 year follow up of
a national birth cohort (The 1946 Birth cohort)
Dr Gita Mishra
MRC 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 members
Professor Michael Wadsworth (Study director)
Professor Diana Kuh
Dr Marcus Richards
Dr Rebecca Hardy
Suzie Butterworth
Stephanie Black
Rachel Cooper
Imran Shah
Warren 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)
Aims
Originally 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 investigating
pathways to physical and cognitive ageing
MRC National Survey of Health & Development
MRC National1946
Survey
of Health
& Development
birth
cohort
1946 birth cohort
Birth Registrations
1 week
(N=16,695)
Birth1946
Registrations
3rd - 9th March 1946 (N=16,695)
Population of the maternity survey
1Population
data collection
atmaternity
birth (N=13,687)
of the
survey
(N=13,687)
Selection of follow-up sample of all single born, married women
with
husbands
in non-manual
employment
Selection
of follow-up
sample and
of allagricultural
single born,
legitimate
andof1fathers
in 4 of in
all non-manual
other comparable
births (N=5,362)
children
and agricultural
employment
20all
data
collections
fromlegitimate
age 2 to 53
years (N=5,362)
and 1 in 4 of
other
single born
children
Study
Study of
of cohort
cohort first
first born
born offspring
offspring
at
ages
4
years
and
8
years
at ages 4 years and 8 years (N=2,205)
(N=2,205)
Study
Study of
of women’s
women’s health,
health, annually,
annually,
at
ages
47-54
years
(N=
at ages 47-54 years (N= 1,572)
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
Maintaining the study’s momentum
Years
Cohort ages
National policy problems
1946
Birth
Costs of maternity, reason for
falling fertility
1947-1950
1-4 years
Social class differences in maternal
and child mortality and morbidity.
Value of health visitors’ work.
1951-1961
6-15 years
Increasing the national level of
educational attainment. The
‘waste of talent’ problem.
1962-1976
16-30 years
Outcomes of education in terms of
occupational choice and skills.
Delinquency.
1976-
30 years onwards
Ageing processes, self care of
health, receptivity to health
promotion.
Sources of information on sample loss
By age 53 years
• Unavoidable losses
– Death
– Emigration
– Living abroad
(n=469 8.7%)
(n=461 8.6%)
(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- Data
ent
Collector
Target
Sample
% achieve
1946 -50
0-4
Mother
HV
4993
95%
1951-61
5-15
Mother &
child
SN or SD or 4900
HV & T
89%
1962-77
16-31
All CMs
P, HV, I
4858
78%
1965-71
19-25
Mothers of RN
first born
1783
94%
1982
36
All CMs
RN
4838
86%
1989
43
All CMs
RN
4826
87%
1993-2000
47-54
Women
P
1999
53
All CMs
RN
84-90%
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
Childhood
1. Shortness at age 4 years
2. Experience of serious illness
3. Late achievement of bladder control
4. Childhood social class of crowding
5. Paternal manual social class, low cognitive test scores
6. Low parental interest in education
7. Teachers’ ratings during adolescence of frequent problems
with discipline, disobedience and aggression
Adulthood
1. Adult social circumstances of low educational attainment
2. 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
1
0.9
0.85
0.8
Low (Q1)
Quartile 2
Quartile 3
High (Q4)
0.75
0.7
0.65
0.6
0.55
Year
96
19
86
19
76
19
66
19
56
19
46
0.5
19
Proportion remaining in study
0.95
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
Highest educational qualifications - degree or higher
Gender
100
80
60
Census
50-54 yrs
NSHD
NSHD
40
Percentage
Percentage
100
80
60
40
20
20
0
0
Males
Census
Ages 45 and up
Males
Females
Social class
Full time employment
100
100
Census
16+ yrs
80
Percentage
Percentage
80
Females
60
40
20
60
40
20
0
0
Non manual
Manual
Non manual
Manual
Males
Females
Sample Representativeness: Comparison of the weighted
sample at age 53 years with 1991(50-54%) census data
Marital Status
100
90
80
Percentage
70
60
50
40
30
20
10
0
Single
Married
Males
Widow ed Separated
/ Divorced
Single
Married
Widow ed Separated
/ Divorced
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; 46:3000-304
Wadsworth MEJ et al. Soc Sci Med 2003; 57:2193-205
Wadsworth MEJ et al. IJE 2006; 35:49-54
Longford NT et al. JRSSA 2000; 163:381-402
De Stavola et al. AJE 2004; 159:671-682
www.nshd.mrc.ac.uk