1. 2001 Census Disclosure Control UK variations
Frank Thomas GROS

2. Disclosure control for 2001 Scotland
Setting a target or average size for output areas (50 households)
Setting a minimum size of areas for key output (e.g. 20 households and 50 residents for Census Area Statistics)
Creating only one set of output areas
Limiting the detail in classifications used in tables
Record swapping before tabulation
Small Cell Adjustment (workplace tables)
Red: UK differences

3. ONS decided to have an average size of around 120 households
Average size for Scottish 1991 output areas was around 55 households So
There would have been much discontinuity in Census geography in Scotland (not a consideration in E&W, NI)

4. ONS increased minimum size of areas (from 20 households to 40)
Not much benefit eg lone Chinese household still a lone Chinese household
FOR
Much discontinuity in geog over 10% OAs would need to be merged

5. Small cell adjustment Record swapping => intruder can’t be sure BUT
ONS worried about perception of identification (1s)
Actual disclosure a matter of 0s not 1s
SCA increases the perceived number of 0s
Decreasing perception of identification increases perception of disclosure AND Upsets users

6. What about 2011? (personal view)
Geographical continuity
No SCA
No record swapping

7. What's wrong with record swapping?
It is ineffective for
population bases other than the geographical variable being swapped
Had to use SCA for workplace tables
populations for geographies within which records are swapped.
Population uniques still at risk in SARs

8. Use Over-imputation
It won't amend the area of residence/enumeration (but we could do record swapping as well perhaps)
BUT
It can focus on particular areas or variables
It can be pegged back a bit for areas or variables where other processing has wrought much change in the data as collected.
It is better than record swapping for non-household populations