Coverage assessment and adjustment methodology

1. Coverage assessment and adjustment methodology Owen Abbott Methodology Directorate, ONS

2. Agenda • Introduction • 2001 One Number Census • 2011 Strategy • The Census Coverage Survey (CCS) • Estimation • Overcount • Adjustment • Summary

3. What is the problem? • Despite best efforts, Census won’t count every household or person • It will also count some people twice • Why is that a problem? • In 2001, we estimated that 3 million persons (6%) missed • Need robust census estimates - counts not good enough • Further problem: • The undercount is not evenly spread • Inner Cities, Deprived areas, Young persons

4. The problem • This is a problem that all census taking countries face • We can try really hard to maximise coverage • But we will still miss households and people • So what do we do? • We must have a robust method for measuring coverage • It must provide accurate estimates at LA level • It must be an integral part of the census process

5. The 2001 Census experience • Estimated 1.5million households missed • 3 million persons missed (most from the missing households but some from counted households) • Subsequent studies estimated a further 0.3 million missed

6. The 2001 Census experience

7. The 2001 Census experience

8. The One Number Census • In 2001, One Number Census methodology was developed • Large Census Coverage Survey • Matching, Capture Recapture, Ratio estimation • Small area estimation to get LA totals • Imputation of missed households and persons • In 2011 we want to build on the ONC, as broadly it was very successful

9. 2011 Aims and Objectives • Measure undercount • Measure overcount • Address lessons from 2001 • Take into account changes • In census design • In the population of interest • Accuracy to be as good or better than in 2001 • 0.2 per cent confidence interval nationally

10. Census Coverage Survey 2011 Census Matching Quality Assurance Estimation Adjustment 2011 Coverage Assessment Overview

11. The Census Coverage Survey • Key component • Similar to 2001 CCS: • Large Sample Survey • 320,000 Households • Sample of small areas (postcodes) • Focus on counting the population • 6 weeks after census day • Short paper based interview • Independent of Census

12. The Census Coverage Survey • Sample design similar to 2001 • Two stage stratified by geography and a ‘hard to count’ index • First sample Output Areas • Then select postcodes within each OA • Sample size determined by optimal allocation • Improvements for 2011 • Sample stratified by Local Authority • More refined HtC index • Better Design variable

13. Estimation • Estimation based on Dual System Estimation • Used mainly for wildlife applications • Requires two counts of the population • Assumptions • Independence • Homogeneity • No matching errors • Applied at very low level

14. Estimation • Use matched Census + CCS data • DSE estimates adjustment for those missed in both Census and CCS Counted By CCS Yes No Counted Yes n11 n10 n1+ By Census No n01n00n0+ n+1n+0n++ DSE count for a postcode: n++ = n1+ n+1  n11

15. Estimation • Generalise sample DSE estimates • Use standard ratio type estimators • Problem – not enough sample in most LAs • Solution – post-stratify LAs into groups • 2011 equivalent of Estimation Areas • Group LAs by type, not geography • Then small area model to get LA estimates

16. Measuring Overcount • Estimate separately • Not yet developed methodology • Sources likely to be: • CCS • Matching Census data • Weight individuals in the DSEs to integrate into estimation methodology

17. Coverage adjustment • Imputation of households and persons estimated to have been missed • Planning on similar process to that in 2001 • Model coverage probabilities • Calibrate weights • Impute households (with people) • Impute persons into counted households • Looking at improvements in modelling steps

18. Summary • Measuring Coverage very important • Integral part of the UK Census process • In the UK looking to build on 2001: • Improvements across the board