All ONS postcode directories are currently based on a file that was originally created by the Department of Transport in the early 1980s for their Regional Highway Traffic Model. Since its introduction, extensive clerical checks have been conducted on data quality and so a high proportion of the postcode administrative area codes and grid references have been manually assigned. Administrative area codes and grid references are usually assigned on the basis of the first address in each postcode. The imputation process depends on the accuracy of the administrative area codes and grid references assigned to postcodes preceding and succeeding a newly introduced postcode and so the process delivers areal references of varying quality which are reflected in status indicators. In a small number of cases it is not possible to impute one or both of the areal references and therefore some fields may be blank for such postcodes.
Indicators are available on postcode directories to determine levels of accuracy.
Many of the other geographies for which postcode links are available are formed of whole electoral wards. Consequently, the quality of these links are dependent on the quality of the ward assignments and, although the other geographies do not have their own status indicators, their individual status is the same as that applied to the electoral ward code. GROS routinely map postcode boundaries and assign all grid references and areal codes clerically to a building judged to be the nearest to the centre of the area covered by the postcode. Postcode directories contain a split postcode indicator which is applicable only to those Scottish postcodes split by local authority boundaries. The indicators are single alphabetic characters, with A always being the largest part, in terms of postal addresses, of a split postcode.