General Register Officefor

S C O T L A N Dinformation about Scotland's people

2001 Census Disclosure ControlUK variations

Frank ThomasGROS

General Register Officefor

S C O T L A N Dinformation about Scotland's people

Disclosure control for 2001Scotland

• 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

General Register Officefor

S C O T L A N Dinformation about Scotland's people

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)

General Register Officefor

S C O T L A N Dinformation about Scotland's people

ONS increased minimum size of areas (from 20 households to 40)

• Not much benefiteg lone Chinese household still a lone Chinese household

FOR

• Much discontinuity in geogover 10% OAs would need to be merged

General Register Officefor

S C O T L A N Dinformation about Scotland's people

Small cell adjustment• Record swapping => intruder can’t be sureBUT• ONS worried about perception of identification (1s)BUT• Actual disclosure a matter of 0s not 1sBUT• SCA increases the perceived number of 0s

Decreasing perception of identification increases perception of disclosureANDUpsets users

General Register Officefor

S C O T L A N Dinformation about Scotland's people

What about 2011?(personal view)

• Geographical continuity

• No SCA

• No record swapping

General Register Officefor

S C O T L A N Dinformation about Scotland's people

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

General Register Officefor

S C O T L A N Dinformation about Scotland's people

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 fornon-household populations