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 sample31,051 households

Stage 2 Households responding to HSE23,132 households

Households non-responding to HSE7,919 households

Stage 3Households containing

1+ age-eligible individual

13,203 householdscontaining 21,193 SM/YP

Households without age-eligible individuals9,929 households

Stage 4Households

dropped401 households

Households containing 1+ living age-eligible

individuals12,802 HHs - 20,764 SM/YP

Stage 5 Households dropped1,224 householdscontaining 1,951

individuals (including 43 dead)

Households permitting re-interview

11,578 householdscontaining 18,813 SM/YP

ELSAsampledefinition

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 issued11,577 HHs, containing

18,824 individuals

New HH91 HHs, containing 96

individuals

All ELSA HH containing 1+ age-eligible individual

11,373 HHs - containing 18,563 individuals

ELSA HH dropped(ineligible)

296 householdscontaining 357 individuals

Responding HH(at least 1 CM/YP/NP

responding)7,935 householdscontaining 12,942

individuals

Non-responding HH3,438 householdscontaining 5,621

individuals

Responding individuals

12,099

CM = 11,391YP = 636NP = 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 issued343 households

HH issued7,591 households

New HH formed

34 households

Stage 3

Ineligible HH175 households

Responding HH6,277 households

Non-responding HH

1,173 households

Stage 4

Responding individuals

9,433

CM = 8,781CP = 57YP = 501NP = 94

Non-responding individuals

420

CM = 120CP = 215YP = 44NP = 41

Ineligible individuals

227

CM (deaths) = 181CM (institutional

moves) = 14CP = 7YP = 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%)

1,989 621

7,168(86%)

1,191 421314

(17%)1,507 168 6 21 517

R

NR

IW1

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

All(Wave 1)

unwtd

XXX XXO XOX XOO

Wave 1 characteristics 11391 7168 1192 314 1291

Age (mean) 65.3 64.1 64.9 64.5 63.5

% Male 46 45 45 46 47

% Black/Asian 2.1 1.3 3.3 5.3 4.1

% London 10 9 11 10 12

% Degree or equivalent 11 13 7 9 8

% No qualifications 44 38 53 55 51

% Highest income quintile 20 23 16 16 18

% Owning property outright 56 58 55 45 49

% Married (1st & only) 56 56 59 53 65

% Retired 50 48 50 41 43

% LLSI 36 32 34 38 35

All(Wave 1)

unwtd

XXX XXO XOX XOO

Wave 1 characteristics 11391 7168 1192 314 1291

% % % % %

Proxy interview 1.39 0.43 1.59 3.18 2.40

Valid NINO given 62 68 56 56 52

Consent to link toGovernment economic data

76 81 70 69 65

Consent to link toGovernment health data

79 84 73 71 69

Completed self-completionquestionnaire

92 95 91 85 86

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 2008-05. Colchester: University of Essex.

Further questions

• s.scholes@natcen.ac.uk