We are often asked why do the ward boundaries in the Local Health GIS data tool, sometimes look different from the actual ward boundaries in an area.

The illustration below shows a screenshot from the Local Health GIS tool where one ward is selected but it appears to be split into two areas. 

Electoral wards, best fit approach

Local Health uses ward boundary maps produced by the Office for National Statistics (ONS).

Geographical outputs from ONS use a standard method based on ‘best-fitting’ output areas (OAs) to larger geographies.

This approach results in small differences to actual ward boundaries in some areas. In other areas, there are more substantial differences, and in a very small number of cases, wards may actually appear to be split in Local Health.

As OAs do not fit perfectly into current ward boundaries, the ONS takes a ‘best fit’ approach. This is done using population weighted centroids, which are summary reference points for the centre of the population in each OA. Where an OA’s centroid falls within a ward's boundary, the OA is assigned to that ward. 

What are output areas?

The ONS produces mid year (June) small area population estimates by Output Areas (OA). Output areas are the lowest level of geographic area specifically designed for census data. These standardised geographic areas are designed to have equal numbers of households or populations within them making it easier to compare output areas. 


They are used primarily in statistical administrative data outputs such as the census and population estimates. Output areas are "nested" within larger geographic areas, such as lower super output area (LSOA), middle super output area (MSOA), and Local Authority areas. that can be summed or aggregated to provide data at these larger areas. 

Best fit wards. An explanation

The reason for this approach is that the majority of administrative data is collected using geographical constructs called output areas (OAs). Although OAs did match ward boundaries in 2001, changes since then mean that OAs no longer fit perfectly into current wards boundaries. The “best fit” approach is consistent with and conforms to the Government Statistical Services Geography Policy, March 2015, which requires statistics produced by government services to

  • Geographically reference statistical events accurately, consistently, and at the lowest possible level
  • Maximize the comparability of their statistics with other official statistics
  • Minimize the impact of changing area boundaries on official statistics outputs
  • Standardise how geography and associated information are defined, used, and presented in statistics
  • Meet legal requirements to safeguard the confidentiality of statistics

What is "best fitting"?

"Best fitting" is the method used to produce estimates for any output geography, by aggregating whole statistical building blocks, even where these may be nested within a higher geography. It is the method used to produce all national and 2011 Census statistics so that estimates produced are consistent, comparable and non-disclosive 

What impact does best-fitting have on statistical estimates?

ONS has previously conducted research into the impact of adopting a best-fit policy which was used to inform the Geography Policy for National Statistics. Overall, the research found that the adoption of best-fit for producing estimates did not have a significant impact, and was an adequate methodology to be used for 2011 Census outputs. Analysis of best-fitting 2001 OAs to 2009 electoral wards found that 94.4 per cent of wards had best-fit estimates that were within 10 per cent tolerance of exact fit estimates to the same ward.


An overview of best-fitting: Building 2011 Census estimates from Output Areas October 2012 Edition No: 2012 v1 Editor: I Coady: Office for National Statistics