The Impact of LFS & APS Reweighting

1. The Impact of LFS & APS Reweighting Marilyn Thomas Labour Force Survey Output Manager, Office for National Statistics

2. Structure • Background to LFS / APS Reweighting • Methodological Improvements to LFS Weighting • Impact of Reweighting on APS Population Estimates • Summary

3. Background to LFS & APS Reweighting • Unreweighted data weighted to: • MYEs up to 2003 & projections based on these MYEs, based on 2001 Census data released in early 2003 • Reweighted data weighted to: • MYEs up to 2006 & projections based on these MYEs, based on revised 2001 Census data released in late 2003

4. LFS/APS Aggregates & Microdata • LFS Published Aggregates – Labour Market Statistics Integrated First Release - interim reweighted • LFS/APS microdata – gov & public datasets - not interim reweighted • By 2007, aggregates out of line with microdata by 800,000 for UK 16+ population • Reweighting – published aggregates and microdata consistent

5. Methodological Changes to LFS (1) • Method & tool for LFS weighting changed for reweighting & ongoing weighting (note: APS already used new method & tool) • Old method & tool – raking ratio estimation, using 3-stage iterative weighting – design-weighted sample raked to known pop totals - local authority district - age-group & sex - region, age-group & sex (LFS User Guide Vol. 1, Section 10)

6. Methodological Changes to LFS (2) • New method & tool – Generalised Estimation System (GES) (Statistics Canada), using a Generalised Regression (GREG) framework. - calibration weighting in a single process, to same population groups as old method. • Advantages - greater efficiency & statistical robustness - facilitates identification & correction of data quality issues

7. Use of LFS & APS Microdata • LFS – robust estimates at UK level • APS – preferred data source for estimates at sub-national level - LFS wave 1 & wave 5 & boost cases in England, Wales & Scotland - 875 interviews with economically active in each UA/LAD (Wales & Scotland) / LEA (England) - 170,000 households & 360,000 individuals - quarterly rolling annual dataset

8. Impact of Reweighting at UA/LAD level • Jan – Dec 2006 APS • Change in 16+ population & employment due to reweighting: total pop / UK born / non-UK born • Figure 1: Change in 16+ population - Range of change in 16+ pop +22% - -6% - 68.2% UA/LADs increase in tot pop 0-5% - 24.4% UA/LADs decrease in tot pop 0-5% - 5.5% UA/LADs increase in tot pop 5-10%

9. Figure 1: Change (%) in 16+ population by UA/LAD

10. Change (%) in 16+ population by UA/LAD: UK born, non-UK born, total UK born non-UK born total • 0 - 5% 67.2% 61.7% 68.2% • 0 - -5% 25.9% 21.9% 24.4% • 5 - 10% 5.5% 10.4% 5.5%

11. Figure 2: Change (%) in 16+ population by UA/LAD - total population

12. Figure 3: Change (%) in 16+ population by UA/LAD – UK born population

13. Figure 4: Change (%) in 16+ population by UA/LAD – non-UK born population

14. Change (%) in employment level by UA/LAD • Figure 5: Change in employment level - Range of change in employ +23% - -6% - 62.7% UA/LADs increase in employ 0-5% - 21.9% UA/LADs decrease in employ 0-5% - 10.4% UA/LADs increase in employ 5-10%

15. Figure 5: Change (%) in employment level by UA/LAD

16. Change (%) in employment level by UA/LAD: UK born, non-UK born, total UK born non-UK born total • 0 - 5% 62.7% 52.2 % 62.7% • 0 - -5% 22.4% 24.9 % 21.9% • 5 - 10% 8.5% 14.4 % 10.4%

17. Figure 6: Change (%) in employment level by UA/LAD – total population

18. Figure 7: Change (%) in employment level by UA/LAD – UK born population

19. Figure 8: Change (%) in employment level by UA/LAD – non-UK born population

20. Summary • Background to LFS Reweighting - population estimates used in unreweighted & reweighted datasets - LFS published aggregates (interim reweighting) & LFS/APS microdata • Changes to LFS weighting methodology & tool • Analysis of impact of reweighting on population estimates at UA/LAD level • Plans for next reweighting programme T.B.C.

21. Note on Sampling Variability Statistical Robustness of APS Estimates 0 ≤ CV < 5 Estimates considered precise. 5 ≤ CV < 10 Estimates considered reasonably precise. 10 ≤ CV < 20 Estimates considered acceptable. CV ≥ 20 Estimates considered too unreliable for practical purposes Note that most of the data included in the analysis meet the ONS criteria for statistical robustness specified above. The exceptions to this rule are for estimates of non-UK born 16+ population and employment in some UA/LADs, due to the small sample sizes of this sub-group at this level of analysis.

22. Useful NS Website References • ELMR (Economic & Labour Market Review): http://www.statistics.gov.uk/StatBase/Product.asp?vlnk=14692&Pos=&ColRank=1&Rank=224 • Labour Force Survey User Guides: http://www.statistics.gov.uk/StatBase/Product.asp?vlnk=1537&Pos=3&ColRank=1&Rank=272 • Labour Force Survey Performance & Quality Monitoring Report (PQM): http://www.statistics.gov.uk/StatBase/Product.asp?vlnk=10675 • Labour Force Survey Summary Quality Report (SQR): http://www.statistics.gov.uk/about/data/methodology/quality/Qualitysocialstats.asp • Labour Market Statistics Integrated First Release: http://www.statistics.gov.uk/statbase/product.asp?vlnk=1944

23. Questions and Comments Thank You for Listening email: marilyn.thomas@ons.gov.uk