The places in our tool have been defined using the ONS built-up area (BUA) geography. This means we have a consistent approach across Wales and it allows us to maximise the data we can show in the tool. However, such an approach means that there will be some ‘places’ which may not be widely recognised as such.
Census data for BUAs in England and Wales, previously known as ‘urban areas’, have been produced every 10 years since 1981. ONS define built-up areas as ‘land which is irreversibly urban in character’, meaning that they are characteristic of a town or city. More information about the methodology used to define BUAs including built-up area sub-divisions (BUASDs) is available here.
At the time of the Census in 2011, 95% of the resident population of England and Wales lived in BUAs, with the smallest BUAs having a population of just over 100, and the largest having a population of nearly 9.8 million. For Wales the figure was 89% of the resident population.
Census data for BUAs are provided on a Census output area best-fit basis. This means that figures are aggregated up from whole output areas that have been best-fit to the areas, rather than the actual figure for the area itself. Consequently, best-fit estimates for areas with a population of greater than 1,500 people are likely to be more robust than the estimates for smaller areas.
ONS suggest “caution is needed when using statistics for built-up areas and sub-divisions with a population of less than 1,500 people”. However, some of the data we want to use is derived from Lower Super Output Area (LSOA) level data (rather than aggregates of output level data). Consequently, we’ve limited the tool to places with a Census population of 2,000 or more. As a result, some small settlements which users might expect to find may not be present in our tool. More information from Welsh Government on the ‘Best fit of Lower Super Output Areas to Built-up Areas’ is available here.