LS attrition: best practice and lessons learned

1. LS attrition: best practice and lessons learned Louisa Blackwell, ONS

2. 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

3. LS Structure Entries 1971-2001 New Births 228,000 Immigrations 122,000 Exits 1971-2001 Deaths 201,000 Embarkations 32,000 1981 534,000 sample members found at 1981 Census 1991 543,000 sample members found at 1991 Census Events: April 1971 to April 2002 Births to sample women 215,000 Births to sample men 49,500 Infant Deaths 2,500 Widow(er)hoods 70,000 Cancer registrations78,000 1971 Original sample: 530,000 members; selected from 1971 Census 2001 540,000 sample members found at 2001 Census

4. Samples available for analysis 1971 1991 2001 1981 536,000 535,000 528,000 1 census 513,000 408,000 2 censuses 420,000 418,000 327,000 3 censuses 331,000 4 censuses 256,000 11,000Emigrations 173,000Deaths Total traced LS members:944,000

5. 10-year and 30-year losses to follow-up

6. Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001 Census sample

7. Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001 Intercensal births Census sample

8. Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001 Intercensal births Census sample Intecensal immigrants

9. 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 birth See Blackwell, L., Lynch, K., Smith, J. and Goldblatt, P. (2003) Longitudinal Study 1971-2001: Completeness of Census Linkage, Series LS No. 10, London: ONS

10. 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

11. Census underenumeration

12. Unobserved embarkation

13. LS attrition through non-linkage: LS tracing at NHSCR

14. 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

15. 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

16. 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