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 ,000 sample members found at 1981 Census ,000 sample members found at 1981 Census ,000 sample members found at 1991 Census ,000 sample members found at 1991 Census ,000 sample members found at 2001 Census ,000 sample members found at 2001 Census Entries New Births 228,000 Immigrations 122,000 Entries New Births 228,000 Immigrations 122,000 Exits Deaths 201,000 Embarkations 32,000 Exits 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

173,000 Deaths 11,000 Emigrations Total traced LS members: 944, , , , , census 408, , ,000 2 censuses 327, ,000 3 censuses 256,000 4 censuses Samples available for analysis

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

Loss to follow-up in each decade, , and Census sample

Loss to follow-up in each decade, , and Census sample Intercensal births

Loss to follow-up in each decade, , and Census sample Intercensal births Intecensal immigrants

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 : 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 CensusEstimated under- enumeration (per cent)

Unobserved embarkation

LS attrition through non-linkage: LS tracing at NHSCR

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: Link had 70 per cent automatch rate - Claimant count linkage had 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