National Records of Scotland

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Appendix 1 - 2001 Census Method

Appendix 1 - 2001 Census Method

GROS Title

The One Number Census process

Following the 1991 Census, it was acknowledged that the census suffered from a degree of undercount, the extent and nature of which was not identified by the 1991 Census Validation Survey. As a result the detailed 1991 Census tables were not consistent with the final estimate of 1991 Census Day population. To avoid a similar situation following the 2001 Census, the Census Coverage Survey (CCS) and the One Number Census (ONC) projects were initiated.

The 2001 CCS was an intensive enumeration of a representative sample of postcodes in Scotland and was designed to be independent of the census and provide the ONC Project with the required data to estimate underenumeration. The ONC project derives its name from the intention to eliminate different population counts so that all census outputs add to one number - the national population estimate that has been adjusted for undercounting. The ONC project ensured that the most appropriate estimation procedures were used to calculate the final census data.

More detail on the conduct of the One Number Census is available.

Comparability with the 1991 Census

Given the approach taken to estimating underenumeration in the 2001 Census, it is not appropriate to compare the 2001 Census results directly with the 1991 Census results. This is because the 2001 Census results cover the whole population but the 1991 Census results did not. A comparison of the coverage achieved in the 1991 and 2001 Censuses is presented in the table below.

Table 1  Comparison of Census coverage in the 1991 and 2001 Censuses


The level of actual census enumeration was similar in 1991 and 2001, 96.9 per cent compared with 96.1 per cent, respectively. But, as the table shows, the 2001 Census results cover the whole population compared with 98.1 per cent coverage in the 1991 Census. The difference in coverage lies in the approach to estimating under-enumeration in the two censuses.

In 1991 the census method allowed enumerators to identify many occupied households where the people within them had not returned a form and include an allowance for them in the census tables. This gave an overall coverage of 98.1 per cent.

The method did not enable enumerators to identify people missed from households where a form was returned nor to make an allowance for any occupied households that they failed to locate. Using data from administrative sources and the 1991 Census Validation Survey it was estimated that these people accounted for about 1.9 per cent of the population. These were not added to the census tables.

In the 2001 Census, the Census Coverage Survey methodology allowed for both types of missing people to be adjusted for in the census tables. Table A1 shows the proportion of people identified as missing and added to the census by sex in each council area.

Census confidentiality

The Registrar General has a legal obligation not to reveal information collected in confidence in the census about individual people and households. Protecting the census data is of key importance and steps have been taken to safeguard confidentiality and protect against disclosure of personal information provided on the census form. Further information is given in 'Scotland's Census - A guide to the results and how to obtain them'. (Adobe Acrobat Portable Document Format (PDF) 4 MB)

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