Introduction
This guide explains the main differences between how postcodes are allocated in our main postcode products:
- Scottish Postcode Directory (SPD), and
- Scottish Statistics Postcode Lookup (SSPL)
What postcode dataset should I use?
It really depends on what you are producing data on and its purpose.
For producing statistics
- You should use the SSPL for all statistical production, to ensure geographic consistency across all official statistics and to support the implementation of the Government Statistical Service (GSS) Geography Policy.
- A key principle of the GSS Geography Policy is that statistics for higher level geographies should be built from statistical building bricks. In Scotland, the statistical building blocks are Output Areas (OAs), Data Zones (DZs), and Intermediate Zones (IZs).
- The SSPL supports the implementation of the GSS Geography Policy by using Output Areas as a building block. Postcodes are allocated to higher geographies based on the postcode’s OA. All the postcodes in an OA will then be allocated to the same higher geography using a best-fit method.
- Using the SSPL ensures that statistics for a higher geography will always be built from OA.
- Using building blocks to create estimates for higher geographies reduces the risk of disclosure of information by differencing. Differencing is the process of subtracting figures for overlapping geographies to reveal information about small populations in the ‘slivers’ that overlap.
- While estimates produced from building blocks reduces the risk of disclosure of information, estimates will be slightly less accurate than if postcode are allocated to higher geographies directly.
For non-statistical operational or administrative purposes:
- SPD lookups should be used if you require the exact location of postcodes, and the associated higher area allocation, i.e., what Electoral ward am I in?
- The SPD allocates postcodes directly to a number of higher geographies using the postcode grid reference.
- In the SPD, postcodes that are in the same OA can be allocated to different higher geographies in cases where an OA falls across two or more of the higher geographies.
- Whilst the SPD method is more exact in allocating postcodes to a higher geography, it increases the risk of disclosure by differencing and so does not support the GSS Geography Policy for National Statistics.
| Information contained |
SPD |
SPPL |
| Non-statistical operational or administrative purpose |
Y |
|
| Production of National or Official statistics |
|
Y |
| Best-fit allocation |
Y |
Y |
| Built from statistical building block (OA) |
|
Y |
| Contains split postcodes |
Y |
|
Summary of Postcode Product Content
SSPL Content
- is provided as one combined table of Small User and Large User postcodes.
- only contains the most recent version of a postcode, there are no duplicates or postcode history.
- there are also no split postcodes (unless they are linked small user postcodes). A split postcode on the SPD is converted to a whole postcode in the SSPL. The original A part of the split postcode is used and any Royal Mail delivery point information, and Census counts are combined to show totals for the whole postcode.
- postcodes are allocated to census output areas using the postcode grid reference, these are then allocated to all higher geographies based on the output area centroid. This is called an output area base.
- there is no postcode boundaries dataset for the SSPL.
SPD Content
- is provided as two tables: Small Users, and Large Users.
- contains full postcode history.
- postcodes are allocated to all higher geographies based on the postcode grid reference.
- this dataset also contains split postcodes. Further information on split postcodes can be found in the NRS Split Postcode policy.
- the SPD is supplemented by postcode boundaries and grid references for live small user postcodes.
SSPL – Making unique postcodes
The Scottish Postcode Directory provides full postcode history (shown in green headed table).
For SSPL the requirement is to have unique postcode records. Only the latest version of the postcode is used (shown in blue headed table).
As the SPD contains split postcodes, some additional work is applied to make the information a whole postcode. When creating a whole postcode, we use the ‘A’ suffix as the basis, this is because the A part of the postcode contains more addresses.
- The postcode drops the ‘A’ suffix.
- If there are Census counts they are added together to produce a whole postcode output.
For example, AB12 3TP was introduced, deleted, and then re-introduced as a split postcode in the SPD (as shown below).
| Postcode |
Date Of Introduction |
Date Of Deletion |
Grid Reference Easting |
Grid Reference Northing |
Split Indicator |
Census Household Count 2022 |
Census Population Count 2022 |
| AB12 3TP |
02-Apr-98 |
17-Apr-00 |
393001 |
799914 |
N |
|
|
| AB12 3TP |
17-Apr-00 |
20-Jun-11 |
392995 |
799905 |
N |
|
|
| AB12 3TPA |
20-Jun-11 |
|
392994 |
799904 |
Y |
1 |
1 |
| AB12 3TPB |
20-Jun-11 |
|
392987 |
800052 |
Y |
0 |
0 |
When converted to SSPL unique records, you can see that the postcode has dropped the ‘A’ suffix while retaining the date of intro and grid references, and Census counts have been added together.
| Postcode |
Date Of Introduction |
Date Of Deletion |
Grid Reference Easting |
Grid Reference Northing |
Split Indicator |
Census Household Count 2022 |
Census Population Count 2022 |
| AB12 3TP |
20-Jun-11 |
|
392994 |
799904 |
Y |
1 |
1 |
Assignment method
It is GSS Geography policy to use statistical building blocks, such as output areas, when aggregating postcode-level data to higher geographies for official statistics.
In each postcode directory the higher area assignment is carried out using ‘Point in Polygon’ method, the only difference is the building brick geography being used.
Using different methods will lead to slightly different statistics.
SSPL postcode assignment method
SPD postcode assignment method
More information on methods of assignment can be found in the Best-fit and Exact-fit Policy on the NRS website.
Differences in output
As previously mentioned, both the SPD and SSPL higher area assignment is carried out using ‘Point in Polygon’ methodology, the only difference is the building brick geography being used. Using different methods will lead to slightly different statistics.
The example below is based on getting a postcode lookup to a 2022 Electoral Ward. If you choose the postcode to higher area [SPD] assignment, the postcode G3 7LW is in Anderston/City/Yorkhill Ward. However, if you use the postcode to output area, output area to higher area [SSPL] assignment, G3 7LW is within 2022 OA S00158775, and the centroid for that OA is in Hillhead Ward.
In a worst-case scenario where the same data is published in two different ways this may lead to disclosure of personal data.
This is why GSS policy states that for statistical purposes we should use one method only. By convention, this is the output area based method.
Producing UK level statistics
NRS Geography are aware that there are comparable products at postcode level produced for the whole of the UK via the Office for National Statistics and Ordnance Survey. Guidance on the Scottish Postcode Directory index and boundaries vs ONS Postcode Directory and Ordnance Survey CodePoint with Polygons is available in the Comparable Postcode Products Information Note on the NRS website.
Similarly, ONS produce a product called the National Statistics Postcode Lookup (NSPL), it relates both current and terminated postcodes in the United Kingdom to a range of current statutory administrative, electoral, health, and other geographical areas via ‘best-fit’ allocation from Census Output Areas in the UK. The NSPL is issued quarterly (Feb/May/Aug/Nov) and is available via the ONS Open Geography Portal
If working with UK or GB level data it is recommended to use consistent products, i.e., NSPL, rather than the SSPL.
Threshold differences between the UK nations
The Census in England, Wales, and Northern Ireland was taken in 2021, whereas in Scotland it was taken in 2022. The main geographies associated with the Census are Output Areas (OA) in England, Wales, and Scotland, and Data Zones (DZ) in Northern Ireland.
The Census geographies are stable over the intercensal period, and are only subject to change following a census, to reflect population and household changes since the previous census.
Further information on how the Census geographies were created are available via the following links:
Consideration should be given to the impacts of the different population threshold size when making comparisons. The table below provides the number of instances and threshold limits per UK nation and Census geography.
| Country |
Geography |
Instances |
Min Population |
Max Population |
Min Household |
Max Household |
| England |
Output Area |
178,605 |
100 |
625 |
40 |
250 |
| England |
Lower Layer Super Output Area |
33,755 |
1,000 |
3,000 |
400 |
1,200 |
| England |
Middle Layer Super Output Area |
6,856 |
5,000 |
15,000 |
2,000 |
6,000 |
| Wales |
Output Area |
10,275 |
100 |
625 |
40 |
250 |
| Wales |
Lower Layer Super Output Area |
1,917 |
1,000 |
3,000 |
400 |
1,200 |
| Wales |
Middle Layer Super Output Area |
408 |
5,000 |
15,000 |
2,000 |
6,000 |
| Scotland |
Output Area |
46,363 |
60 |
n/a |
25 |
90 |
| Scotland |
Data Zone |
7,392 |
500 |
1,000 |
n/a |
n/a |
| Scotland |
Intermediate Zone |
1,334 |
2,500 |
6,000 |
n/a |
n/a |
| Northern Ireland |
Data Zone |
3,780 |
100 |
800 |
50 |
400 |
| Northern Ireland |
Super Data Zone |
850 |
1,000 |
3,000 |
500 |
1,300 |
Base Postcode Data
The base postcode data used in the creation of the SSPL and NSPL is the SPD and the ONSPD respectively. There are a very small number of Scottish postcodes that exist on one directory but not the other, and vice versa.
The differences between the SPD and ONSPD are detailed in our Comparable Postcode Products Information Note available on the NRS website.
We are also aware that for a very small percentage of postcodes the allocation of 2022 Output Area differs between the postcode products that NRS and ONS release, more information on this is available in the Comparable Postcode Products: Census Output Area Allocation Differences Information Note on the NRS website.
Queries
For changes to the specification for the SSPL please contact the SG Office of the Chief Statistician.
Queries on the data held in the SSPL please contact the NRS Geography Customer Services team.
