ls attrition: best practice and lessons learned louisa blackwell, ons
TRANSCRIPT
LS attrition: best practice and lessons learned
Louisa Blackwell, ONS
Introduction
• Structure of the ONS Longitudinal Study• Samples available for analysis• LS losses to follow-up; change over time• Impact of loss to follow-up on analysis• Explaining LS loss to follow-up • Minimising attrition through design• Minimising attrition through best practice in data
processing• Treatment of attrition in LS analysis
LS Structure
1971
Original sample: 530,000
members; selected from 1971 Census
1971
Original sample: 530,000
members; selected from 1971 Census
1981
534,000 sample
members found at 1981
Census
1981
534,000 sample
members found at 1981
Census
1991
543,000 sample
members found at 1991
Census
1991
543,000 sample
members found at 1991
Census
2001
540,000sample
members found at 2001
Census
2001
540,000sample
members found at 2001
Census
Entries 1971-2001New Births 228,000Immigrations 122,000
Entries 1971-2001New Births 228,000Immigrations 122,000
Exits 1971-2001Deaths 201,000Embarkations 32,000
Exits 1971-2001Deaths 201,000Embarkations 32,000
Events: April 1971 to April 2002Births to sample women 215,000Births to sample men 49,500Infant Deaths 2,500Widow(er)hoods 70,000Cancer registrations 78,000
173,000 Deaths 11,000 Emigrations
Total traced LS members: 944,000
536,000528,000 535,000513,000
1971 20011981 1991
1 census
408,000
420,000
418,000
2 censuses
327,000
331,000
3 censuses
256,000 4 censuses
Samples available for analysis
10-year and 30-year losses to follow-up
Losses over 30 years
History Total traced LS members
Deaths and embarkations 1971-2001
Matched at 2001 Census
Intercensal loss (per cent)
Present 1971 Census
513,000 184,000 256,000 14
Losses over 10 years
History Total traced LS members
Deaths and embarkations 1991-2001
Matched at 2001 Census
Intercensal loss (per cent)
Present 1991 Census
537,000 60,000 419,000 11
1990s births 71,000 1,000 60,000 14
1990s immigrants
46,000 2,000 15,000 63
Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001
Census sample
0
10
20
30
40
50
60
70
1971-1981 1981-1991 1991-2001
Census
per
cen
t
Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001
Census sample Intercensal births
0
10
20
30
40
50
60
70
1971-1981 1981-1991 1991-2001
Census
per
cen
t
Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001
Census sample Intercensal births Intecensal immigrants
0
10
20
30
40
50
60
70
1971-1981 1981-1991 1991-2001
Census
per
cen
t
Impact of loss to follow-up on analysis
• Introduces significant error in all survival analysis • Biases all analyses of sub-groups; those lost to follow-up
are disproportionately:
-young and male
-immigrants
-living in London, particularly Inner London
-in a minority ethnic group
- not married
- born to a young mother in the 1990s
- sole registered at birthSee Blackwell, L., Lynch, K., Smith, J. and Goldblatt, P. (2003) Longitudinal Study
1971-2001: Completeness of Census Linkage, Series LS No. 10, London: ONS
Explaining LS loss to follow-up
Census underenumeration• LS members resident in England and Wales but
not enumerated by Census
Unobserved embarkation• LS members no longer resident in England and
Wales at Census but their emigration was not previously recorded in the LS
LS non-linkage• LS linkage failure, through mis-recording of date
of birth or poor quality person identifiers
Census underenumeration
Census Estimated under-enumeration (per cent)
1971 0.40-1.00
1981 0.45
1991 1.90
2001 6.00
Unobserved embarkation
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
Year
Rat
io -
LS
/IPS
Males Females
Ratio of LS emigrants to International Passenger Survey outflows from England and Wales, 1971-2000
LS attrition through non-linkage: LS tracing at NHSCR
Tracing rates by census year and sex
Census Males(%)
Females(%)
Persons(%)
1971 97.2 96.5 96.81981 98.8 98.9 98.81991 98.3 98.5 98.42001 99.1 99.5 99.3
1. Source: Office for National Statistics Longitudinal Study (LS)
Minimising attrition through design
• No respondent burden as a result of the LS• Comprehensive recording of deaths (deaths
abroad are under-recorded)• Use of NHSCR for tracing allows cross-validation
and avoids double counting • Entry to the LS is only permitted through birth,
immigration and Census- example of quality management is provided by CCC retry routines
• Intelligent matching
Minimising attrition through processing
• Electronic data capture for 2001• Use of Census images to aid tracing in 2001• Combining automatic with operator matching:
- 2001 Link had 70 per cent automatch rate
- Claimant count linkage had 88-90 per cent automatch rate
• Query resolution between processing and tracing teams
• Dedicated teams drawing on 35 years’ of experience in matching LS data
Treatment of attrition in analysis
• Restrict analysis to LS members traced at NHSCR with matched Census records
• Investigate characteristics of LS members lost to follow-up to understand bias in analysis
• Validate cross-sectional distributions with 100 per cent census data
• For survival analysis:
- Make assumptions about timing and characteristics of those lost, based on available evidence