Coverage assessment and adjustment methodology

Owen Abbott

Methodology Directorate, ONS

Agenda

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

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

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

The 2001 Census experience

• Estimated 1.5 million 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

The 2001 Census experience

Underenumeration of Census by agegroup

0.0%

2.0%

4.0%

6.0%

8.0%

10.0%

12.0%

14.0%

16.0%

0 1-4 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49 50-54 55-59 60-64 65-69 70-74 75-79 80-84 85+

Agegroup

ON

C/C

en

su

s

Males Females

The 2001 Census experience

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

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

2011 Coverage Assessment Overview

Estimation

Matching

Adjustment

2011 Census

Quality Assurance

Census Coverage

Survey

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

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

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

Estimation

• Use matched Census + CCS data• DSE estimates adjustment for those missed

in both Census and CCSCounted By CCSYes No

Counted Yes n11 n10 n1+

By Census No n01 n00 n0+

n+1 n+0 n++

DSE count for a postcode:

n++ = n1+ n+1 n11

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

Census

DSE

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

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

Summary

• Measuring Coverage very important• Integral part of the UK Census process• In the UK looking to build on 2001:

- Improvements across the board