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2001 Census

Disclosure Control

There is a legal obligation not to reveal information collected in the Census about individuals and households. In presenting detailed results, protecting this data is of key importance. Disclosure is prevented by a combination of methods.

A - Setting a target or average size for output areas (50 households)
B - Setting a minimum size of areas for key output (e.g. 20 households and 50 residents)
C - Creating only one set of output areas (two sets of overlapping output areas could be 'differenced' to create unintended below - threshold areas)
D - Limiting the detail in classifications used in tables
E - Record swapping before tabulation
F - Small Cell Adjustment (workplace tables)

Methods A to D ensure that in only a limited number of cases households or persons in one of the categories of a variable in a table belong to a single category in another variable. When this happens, information can be disclosed from the table about those households or persons. For example, if there were only one Chinese person in an output area, a table for that output area tabulating ethnicity (with 'Chinese' as a category) by employment status would reveal that person's employment status. Method E, record swapping, is the measure employed that ensures that this cannot be assumed. A swapped record is tabulated in a different output area from where the data was collected. This does not materially affected the overall results.

A small number of tables have been subject to Method F whereby cells containing small numbers were adjusted randomly. These tables are those on OAs as place of workplace rather than place of residence. Methods B & E are ineffective for data on Workplace.

You should note that there is some degree of error in Census information as there is any large scale collection of data. This applies particularly to counts of people travelling to place of work or study by destination or counts of migrants by previous address. The accuracy of such counts cannot be controlled in the same way as counts of residence. General information about Data Quality may be found on this website.

One of the conditions of using Census data is that users do not attempt to obtain or derive information about a specific individual or household and not to claim to have obtained or derived such information.

All three UK Census offices use these methods to some extent.