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 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 540,000 sample members found at 2001 Census 2001 540,000 sample members found at 2001 Census Entries 1971-2001 New Births 228,000 Immigrations 122,000 Entries 1971-2001 New Births 228,000 Immigrations 122,000 Exits 1971-2001 Deaths 201,000 Embarkations 32,000 Exits 1971-2001 Deaths 201,000 Embarkations 32,000 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 registrations 78,000

4. 173,000 Deaths 11,000 Emigrations Total traced LS members: 944,000 536,000 528,000 535,000 513,000 19712001 1981 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

5. 10-year and 30-year losses to follow-up Losses over 30 years HistoryTotal traced LS members Deaths and embarkations 1971-2001 Matched at 2001 Census Intercensal loss (per cent) Present 1971 Census 513,000184,000256,00014 Losses over 10 years HistoryTotal traced LS members Deaths and embarkations 1991-2001 Matched at 2001 Census Intercensal loss (per cent) Present 1991 Census 537,00060,000419,00011 1990s births71,0001,00060,00014 1990s immigrants 46,0002,00015,00063

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 Census sample Intercensal births

8. Loss to follow-up in each decade, 1971-81, 1981-91 and 1991-2001 Census sample Intercensal births 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 CensusEstimated under- enumeration (per cent) 19710.40-1.00 19810.45 19911.90 20016.00

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