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Occasional Paper

3.2 Geography

Geography in the Dataset

Consultation with users indicated that small area population estimates produced for postcode sectors and 1999 wards would satisfy most user requirements. It is intended to provide population estimates for both geographies. However, this dataset only provides population estimates for postcode sectors as GROS has not yet finished digitising the 1999 ward boundaries that are needed to map postcodes to wards. When the boundaries become available, expected summer 2001, the 1999 small area population estimates will be updated to include 1999 ward estimates.

These geographies are determined by matching the patient's current postcode from the CHI against the GROS Postcode Index 99/2[Footnote 1]. A computer file derived from GROS digitised postcode boundaries, which provides an allocation of live and deleted small and large user postcodes within Scotland to higher areas such as electoral and administrative areas and other information including 1 metre national grid references.. This match determines if a postcode is valid and provides area codes (council areas, health board areas, etc.) for each valid postcode.

Postcode sectors are not contiguous with health board or council boundaries. Therefore, postcode sectors have been split to nest within health board and council boundaries. Where postcode sectors are split, a naming convention has been used to identify that it is a split postcode sector and which identifies the health board and council area the split postcode sector lies within. For example taking the fictitious split postcode sector of KY15 7xxYY - 'xx' is a number between 01 and 15 representing the health board; and, 'YY' is a number between 01 and 32 representing the council area. Codelists for the health boards and council areas are provided in Annex D.

Postcode quality

Approximately 99 per cent of the postcodes on the CHI were found to be valid. As the remaining one per cent of records were considered to be part of the currently registered population, they had to be assigned postcodes or area markers to enable them to be used in the estimation process. This assignment was done using a donor imputation method, which assumes that patients with similar characteristics will live in similar geographic locations.

For each patient record without a valid postcode, a similar record with a valid postcode is selected at random. The similarity of records was determined by comparing details of GP practice code, age, sex and health board of residence. The record with the valid postcode is the donor and its postcode (and subsequent area codes) is assigned to the record with the invalid postcode.

Footnote

1. A computer file derived from GROS digitised postcode boundaries, which provides an allocation of live and deleted small and large user postcodes within Scotland to higher areas such as electoral and administrative areas and other information including 1 metre national grid references