1. Peter Lynn ESDS Event, 12 March 2004 Weighting for Longitudinal Surveys Peter Lynn University of Essex, UK

2. Peter Lynn ESDS Event, 12 March 2004 What is different about longitudinal surveys? Multiple combinations of waves: for which responding sample(s) do we want weights? Definition of longitudinal population – implications for weighting targets Different nature of non-response: should influence how we construct weights Data from previous wave(s) available as potential weighting variables Special case where new entrants after wave 1 are possible

3. Peter Lynn ESDS Event, 12 March 2004 What is not different? Same issues to be addressed: design, sampling error, non- response error Same arguments regarding choice of weights (but different context) Same issues regarding estimation and standard errors

4. Peter Lynn ESDS Event, 12 March 2004 Patterns of response Wave:1234 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

5. Peter Lynn ESDS Event, 12 March 2004 Example: YCS (Cohort 5) Wave:123 No. of units % of units 18,39644.9 22,55513.7 32,66014.2 49004.8 53862.1 65753.1 73521.9 82,85515.3 (Lynn et al, 1994)

6. Peter Lynn ESDS Event, 12 March 2004 Which combinations of waves to weight? I There are c possible combinations: where n is the number of waves ncnc 239511 37101023 415112047 531124095 663138191 71271416,383 8255

7. Peter Lynn ESDS Event, 12 March 2004 Which combinations of waves to weight? II Key/ commonly-used combinations Similarity of response propensities Small differences in numbers of sample units YCS example: Weights provided for X??, XX?, XXX

8. Peter Lynn ESDS Event, 12 March 2004 How are weights calculated? I As usual: components for sample design, non-response, PS Sample design component (usually) does not change Hence, typically use wave 1 (CS) weights and adjust for subsequent non-response But response now means being in relevant longitudinal sample

9. Peter Lynn ESDS Event, 12 March 2004 How are weights calculated? II Components of wave 1 weight: Adjustment for wave 2 NR:

10. Peter Lynn ESDS Event, 12 March 2004 How are weights calculated? III At wave 3, there are 2 choices: 1. Adjust the wave 2 weight: 2. Adjust the wave 1 weight: 1.More sensitive to changing nature of NR? More available auxiliary data. 2.Better prediction likely (conditional upon data). Less random variation in weights,

11. Peter Lynn ESDS Event, 12 March 2004 Non-response bias ornr yy n o yy

12. Peter Lynn ESDS Event, 12 March 2004 Reminder: residual non- response error Implication: Use of survey data from previous wave to construct adjustment weight can help to produce small

13. Peter Lynn ESDS Event, 12 March 2004 Example: BEPS 1992-97 Wave 1 NR: Age u-25, 75+, did not vote, self-employed, unemp Wave 2 attn: Lack of interest in politics, low political knowledge scores, did not vote, 60+ Wave 3/4 attn: u-25, never married, sick/retired, low quals (Taylor, Heath & Lynn, 1997)

14. Peter Lynn ESDS Event, 12 March 2004 Special Case: New Entrants Population entrants vs. sample entrants Not dealt with by wave 1 design weights plus adjustment for attrition: need different approach May need to estimate seln prob.s Probabilities may be conditional e.g. Household panels: fair shares methods

15. Peter Lynn ESDS Event, 12 March 2004 Choosing and using weights Familiarise yourself with available weights Choose carefully Give feedback to data producers Consider developing your own weights (esp. post-w1): some advantages Alternatively identify correlates and include as covariates/ controls

16. Peter Lynn ESDS Event, 12 March 2004 Weighting for Longitudinal Surveys Peter Lynn University of Essex, UK

17. Peter Lynn ESDS Event, 12 March 2004 References Lynn P (2004) Sample survey weighting, in Kempf-Leonard K. (ed.), Encyclopedia of Social Measurement, Academic Press. Lynn P, Purdon S, Hedges B & McAleese I (1994) England and Wales Youth Cohort Study: An Assessment of Alternative Weighting Strategies, Employment Department Research Series, YCS Report no. 30. Taylor B, Heath A & Lynn P (1997) The British Election Panel Study 1992-1997: response characteristics and attrition. Working Papers of the Centre for Research into Elections and Social Trends (CREST), paper no. 40, University of Strathclyde.