What are the differences between the ONSPD and the NSPL? Introduction This paper explains the differences in the allocation of postcodes in the ONS Postcode Directory (ONSPD) and the National Statistics Postcode Lookup (NSPL) products. To summarise: ONS Postcode Directory (ONSPD) – assigns individual postcodes separately to each higher level geography National Statistics Postcode Lookup (NSPL) – assigns individual postcodes to Output Areas (OAs), and then using its assigned OA to lookup its higher level geography. Methodology: ONSPD The ONSPD primarily associates postcodes with a number of other geographies. The postcode is allocated to each type/level of geography using a point-in-polygon method. This places each unique postcode in the actual boundary of the geography it falls in. Method: The diagram shows that for each of the different higher level geographies in this diagram (output areas, wards and parliamentary constituencies) the postcodes are allocated using a point in polygon process each time. Example: To clarify, the example below shows a number of postcodes (red dots) within the same Output Area but straddles 2 different Westminster Parliamentary Constituency (ParliCon) boundaries. The postcodes are allocated directly to the ParliCon based on where the postcode geographically falls within (the point-in-polygon method). Zooming in 00MGNW0012 Therefore the postcodes are allocated to the 2 different ParliCon boundaries (C94 and C95) as shown below, regardless of the fact they are within the same output area: Postcode MK13 9HB MK13 9AP MK13 9HF MK13 9HG MK8 8DD MK8 8ND MK8 8DF ParliCon code C94 C94 C94 C94 C95 C95 C95 Output Area 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 Although this method is accurate in that postcodes are assigned to the correct geographical higher level geography, it does not support the Geography Policy for National Statistics. This is why ONS Geography is now providing a product that does support the policy – the NSPL. NSPL: A key principle of the Geography Policy for National Statistics is that statistics for higher level geographies should be built from “small area stable statistical building bricks”. These are Output Areas, or Super Output Areas if not available at OA. The purpose of the policy is that national statistics are geographically consistent and comparable. The policy method recommends first linking postcodes to the Output Areas (using the ONSPD point-in-polygon method). Postcodes should then be linked to all higher geographies by using an Output Area to higher geography lookup. The Output Area to higher geography lookup is created by ONS “best-fitting” 2001 Census population data for each Output Area into each higher level geography. Where the higher level geography splits an Output Area, that OA is best-fitted to the higher geography that the greatest proportion of that OA’s population falls in. If the whole OA is within one area in the higher geography then this is assigned to that area immediately. Method: The diagram shows that postcodes are allocated to Output Areas based on the point in polygon process. The Output Areas are then allocated to each type/level of geography based on the population of the Output Area (where the greater proportion lies, or if split between more than two higher geographies, the greatest proportion of population). Example: To clarify, the example below shows a number of postcodes (red dots) within the same Output Area but straddles 2 different Westminster Parliamentary Constituency (ParliCon) boundaries. The postcodes are allocated to the ParliCon boundary based on where the greatest proportion of the population of the Output Area falls within. 31 18 25 20 24 45 Zooming in 65 00MGNW0012 The population of the Output Area (00MGNW0012) that falls within ParliCon C94 is 94 (31 +18+25+20) and the populations that falls within ParliCon C95 is 134 (24+45+65). Therefore all the postcodes within this Output Area are allocated to the ParliCon C95 as it contains the greater proportion of the population. Postcode MK13 9HB MK13 9AP MK13 9HF MK13 9HG MK8 8DD MK8 8ND MK8 8DF ParliCon code C95 C95 C95 C95 C95 C95 C95 Output Area 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012 00MGNW0012