The English Longitudinal Study of Ageing (ELSA) Sample design & response Shaun Scholes NatCen

Content Presentations on weighting are incredibly dull (!) so: A focus on sample design:  Who’s included?  Who’s excluded and why? A brief analysis of response by Wave 3 (for those who took part at Wave 1):  Who remains?  Who drops-out?

Sample design (1) – Eligibility (core members) Include if belong to ELSA’s target population:  Persons aged 50+ (age-eligibility)  Living in private households in England (wave 1) and Great Britain (subsequent waves) –Institutional interviews  Partners interviewed but not of interest in themselves: –‘End-of-life’ interview –Understand circumstances of couple –Understand circumstances after ‘split’

Age-eligibility for ELSA (core members)

Sample design (2) – issued for fieldwork Being eligible (i.e. in target population) does not guarantee being issued for fieldwork Respondents at HSE/each ELSA wave could have refused permission to be re-contacted:  BUT we use a household rather than individual definition  Individual ‘refusers’ still have an opportunity to take part if another eligible person in the household did not refuse re-contact

Origin of the ELSA sample ELSA is a follow-up of Health Survey for England responding households:  Benefits of follow-up: –Nationally representative of private households –Identify eligible individuals at reasonable cost –Wide range of information already collected –Respondents took part in survey already so more likely to take part in new survey  Disadvantages of follow-up: –Initial non-response –Refusals to be re-contacted for further study –Drop-out between initial and follow-up survey

Composition of ELSA sample Determined by:  Being in a responding HSE HH  Eligibility: –Age (Cohort 1 core members born before 1 March 1952) –Living in private household in England at time of wave 1 –Being a partner of sample member  Whether issued for fieldwork: –Age-eligible person in HH agreeing to further contact post HSE  Propensity to respond conditional on being issued: –Household level non-response –Individual level non-response within responding households –Item/module non-response Self-completion questionnaire (all waves) Income information (all waves) Nurse visit and blood sample (waves 2 & 4)

Stage 1 HSE sample 31,051 households Stage 2 Households responding to HSE 23,132 households Households non- responding to HSE 7,919 households Stage 3 Households containing 1+ age-eligible individual 13,203 households containing 21,193 SM/YP Households without age- eligible individuals 9,929 households Stage 4 Households dropped 401 households Households containing 1+ living age-eligible individuals 12,802 HHs - 20,764 SM/YP Stage 5 Households dropped 1,224 households containing 1,951 individuals (including 43 dead) Households permitting re- interview 11,578 households containing 18,813 SM/YP ELSA sample definition SM - Age-eligible sample member YP- Young partner

Response at W1 CM – core member YP – young partner NP – new partner Stage 6 Stage 7 Stage 8 Stage 9 HH issued 11,577 HHs, containing 18,824 individuals New HH 91 HHs, containing 96 individuals All ELSA HH containing 1+ age-eligible individual 11,373 HHs - containing 18,563 individuals ELSA HH dropped (ineligible) 296 households containing 357 individuals Responding HH (at least 1 CM/YP/NP responding) 7,935 households containing 12,942 individuals Non-responding HH 3,438 households containing 5,621 individuals Responding individuals 12,099 CM = 11,391 YP = 636 NP = 72 Non- responding individuals 502 Individuals dropped (ineligible) 340

Response tree from wave 1 to wave 2 CM Core member CP Core partner YP Young partner NP New partner (entering study at waves 1 and/or 2) Stage 1 All HH containing at least 1 CM, YP or NP responding in wave 1 7,934 households Stage 2 HH not issued 343 households HH issued 7,591 households New HH formed 34 households Stage 3 Ineligible HH 175 households Responding HH 6,277 households Non-responding HH 1,173 households Stage 4 Responding individuals 9,433 CM = 8,781 CP = 57 YP = 501 NP = 94 Non-responding individuals 420 CM = 120 CP = 215 YP = 44 NP = 41 Ineligible individuals 227 CM (deaths) = 181 CM (institutional moves) = 14 CP = 7 YP = 25

Response over three waves Focus here on Cohort 1 core members (took part at wave 1) The next slide shows, at each wave, the number of:  R ~ Respondents  NR ~ Non-respondents –refusals –Non-contacts –Unable to trace –‘Other’ (e.g. ill/away during fieldwork)  I ~ Ineligible cases (known ineligibility)

11,391 8,780 (82%) 7,168 (86%) 1, , (17%) 1, R NR I W1 W2 W3

Non-response Non-response causes two problems for longitudinal surveys (Uhrig, 2008):  Lower sample size results in lower precision  Non random non-response means sample becomes unrepresentative as the longitudinal sample ages. Bias exists when the R and NR vary with respect to outcomes Advantage of panel surveys (compared to cross- sectional ones) is that we have survey data collected at the first wave (Lynn, 2008) to compare R and NR - and use for weighting.

Predictors of non-response Four groups of eligible cases are compared across selected W1 variables:  XXX (took part in all 3 waves)  XXO (dropped out after W2)  XOX (returned at W3 after missing W2)  XOO (dropped out after W1) All wave 1 respondents used as the benchmark. If NR = random all distributions would equal the W1 distribution (Lynn et al., 1994) Hypothesis:  XXX = most educated/affluent, younger, healthier  XOO = least educated/affluent, older, poorer health  XOX/XXO = somewhere in between

Conclusions Many steps to go through to have been selected for the ELSA sample and to have taken part in all three waves. Be aware of the existence of different cohorts as time progresses. Be critical/cautious about the types of respondents who remain - a random subset of the target population? Probably not.

References Lynn P (2008) ‘Non-response’ in E.De. Leeuw, J.J. Hox and D.A. Dillman International Handbook of Survey Methodology (New York: Lawrence Erlbaum Associates). Lynn, P., Purdon, S., Hedges, B. and McAleese, I. (1994), The Youth Cohort Study: An Assessment of Alternative Weighting Strategies and their Effects, Employment Department Research Series YCS. Report no.30. Uhrig, S.C. Noah (2008) ‘The nature and causes of attrition in the British Household Panel Survey’, Working Papers of the Institute for Social and Economic Research, paper Colchester: University of Essex.

Further questions