1. Towards a Fully Adjusted Census Database for the 2011 Census
Christine Sexton (ONS)
Alan Taylor (ONS)
James Brown IoE)

2. Outline
Overview of the Census Coverage Assessment and Adjustment Strategy for 2011
The 2001 Adjustment Strategy
Learning from 2001
Assessment of the 2001 Adjustment System
The Way Forward

3. Census Coverage Survey
Overview of the Coverage Assessment and Adjustment Process
Estimation
Matching
Adjustment
2011 Census
Quality Assurance
Census Coverage Survey

4. The 2001 Adjustment Strategy
Stage 1: Imputation of missed households (with people)
Model to derive predicted census household coverage probabilities using matched census to CCS data to obtain coverage weights
tenure, ethnicity, household structure
Calibrate coverage weights to key variable estimates
tenure exactly
Impute households with people into the database
whole household records copied

5. The 2001 Adjustment Strategy
Stage 2: Imputation of missed individuals into counted households
Model to derive predicted person within counted census household coverage probabilities using matched census to CCS data
age, sex, activity, household structure, LA
Calibrate coverage weights to key variable estimates at local authority level
age-sex groups exactly
Impute people into census counted households
whole person records copied

6. The 2001 Adjustment Strategy
Stage 3: Final adjustment
Further adjustments to meet local authority level estimates for age-sex groups and household size distributions
taking out imputed individuals
putting in extra individuals
(pruning and grafting)
Ref: Steele, Brown and Chambers (2001), JRSS, series A.

7. Learning From 2001
Insufficient control of household size and characteristics for imputed households
Too many people in certain age-sex groups added at household imputation stage
Much time spent “pruning and grafting”
Insufficient heterogeneity in the imputed population for some characteristics
Whole records copied to imputed households and individuals
Ensured Census edit rules satisfied but may not reflect variability in population

8. Assessing the Performance of the 2001 System
Used simulations
Uses 2001 census extracts as the ‘true population’
modelled 2001 matched census and CCS data
10 simulated censuses and CCSs for one Estimation Area (two LAs)
Census coverage 94%
200,000 households
490,000 persons
Used true totals as calibration constraints
LA age-sex group totals, activity, tenure, household size

9. Performance measures

10. Relative Average Bias Results for Tenure

11. RRAMSE Results for Tenure

12. Relative Average Bias Results for Males by Age

13. RRAMSE Results for Males by Age

14. Relative Average Bias Results for Activity

15. RRAMSE Results for Activity

16. (pruning and grafting)
The Way Forward
Aim to improve imputation by gaining better control of numbers of individuals imputed into households and their characteristics
Correct distribution of age group and household size at lower levels of geography
Reduce time spent on final adjustment
(pruning and grafting)

17. Modelling Missed Individuals
In 2001 we modelled individuals missed within counted households
no direct control of individuals missed within missed households
Proposed new model –
all missed individuals in single model
missed within counted households
missed within missed households
Calibrate coverage weights for all individuals
then split weights into two components based on the model

18. Reverse the order of imputation
In 2001 household imputation carried out first
Within household imputation used to make up shortfall
Household weights did not match individual totals
Imputed households did not contain correct types of individuals

19. Reverse the order of imputation
New person model gives direct control over split between two sources of undercount
Can put missed individuals into counted households first to complete counted households
Then model census household coverage
Calibrate household weights to key variables at EA level – tenure and household size
Also calibrate household weights to key individual level variables from the persons in missed HHs totals – age-sex groups – at LA level to recover totals at the individual level

20. Conclusions
By implementing the proposed changes we aim to improve on the 2001 system by gaining better control of the age-sex by household size distribution of the adjusted database and reduce the need for the final stage adjustment
Analysis of 2001 method gives us a bench-mark to compare changes
Work in progress

21. Questions?