FOI Ref: FOI/2023/4863
You asked
I am writing and hope to seek professional advice on UK postcodes and wards issues when referring to the UK census 2001 and 2011.
I wonder if there have been any changes in the postcodes listed in the file "2 nspd_2011_05-postcode+ward.xls" (columns pcd and pcd2) since 2000 and if these postcodes indicate the same addresses in the UK census 2001 and the UK census 2011.
As you published on the website, there are two main postcode products, the NSPL and NOSPD. I wonder if the wards shown in columns casward, osward and statsward are all based on the NOSPD method, which means postcodes are directly allocated to related wards using the postcode's 1-metre grid reference. If this is the case, I guess these three columns indicate the same geographic areas in 2001 and 2011.
In addition, I know that the column 'casward' indicates the wards' codes in Census 2001, and the column 'osward' indicates the wards' codes in Census 2011, but I am not sure about the difference between casward and statsward. I really appreciate it if you could kindly make me distinguish between them.
We said
Thank you for your freedom of information request.
Of your 65,535 listed postcodes, 18,438 postcodes have been discontinued since 2000, and so have changed. These discontinued postcodes may result from buildings becoming demolished, or through Royal Mail reorganisation -- resulting in a postcode for an address becoming changed.
The addresses recorded in both the 2001 and 2011 Census which have the same postcode will not necessarily be exactly the same. The reason for this is that over time dwellings (plus other buildings) may get demolished, and new dwellings built. Some of these new dwellings will be allocated the same postcode as existing nearby dwellings. Whilst over time therefore a postcode may remain constant, the addresses relating to that postcode may change.
The way postcodes in our two postcode directories are allocated to wards differs. With the National Statistics Postcode Lookup (NSPL) postcodes are allocated in England and Wales to a small area geography - Output Areas, and then referenced to higher geographies, including wards, by an Output Area best-fit methodology.
The ONS Postcode Directory (ONSPD) allocates postcodes to other geographic areas differently, by using a grid reference location for each postcode and a point-in-polygon methodology to assign this grid reference to a higher geography.
Postcodes will generally be allocated to the same ward with both directories, but some postcodes will be allocated to different wards, reflecting the different methodology used to allocate postcodes to higher geographies with the two directories. For more information please see information on our website Postcode products.
Census outputs for 2001 and 2011 wards were produced using an Output Area best-fit methodology, and is therefore consistent in terms of methodology with the postcode to higher geography allocations with the NSPL.
Census Area Statistics (CAS) wards 'casward' were created for 2001 Census outputs. In England and Wales they were identical to the 2003 statistical wards 'statsward', except that 25 of the smallest (sub-threshold) wards were merged into 7 'receiving' wards to avoid the confidentiality risks of releasing data for very small areas. This happened to those wards with fewer than 100 residents or 40 households (as at the 2001 Census). There were a total of 8,850 CAS wards in England and Wales, 18 fewer than the total number of 2003 statistical wards.