Revising small area statistics geographies – data zones and intermediate zones: consultation

Methodology

This section outlines the methodology we have used to update Data Zones and Intermediate Zones.

Boundaries

The method which was used to update 2001 Data Zone boundaries following the 2011 Census has been replicated for this Census 2022 update. The aim for this update was to apply Census 2022 data, re-align to local authority boundaries (as at Census Day 2022), and account for changes in population, while keeping boundaries relatively comparable to those created for 2011. The criteria used in the definition of 2022 Data Zones were:

• That they be built up from 2022 Census Output Areas.
• Maintain approximately equal household resident populations of 500 to 1,000 people, with an absolute minimum of 375 people and a maximum of 1,125.
• Be a continuous area without multiple extents, unless caused by water (e.g. Data Zones including islands).
• Where possible, changes from 2011 occur within Intermediate Zone boundaries.
• Maintain a compactness of shape.

Initially, a first draft of boundaries was created by aggregating 2022 Census Output Areas to 2011 Data Zones, based on the location of the Output Area Population Weighted Centroids. The population (defined as household residents) was summed for each 'best-fit' Data Zone to assess whether it fell within the required population thresholds. Data Zones which had seen a decrease in population (e.g. as the result of housing demolition) and fell below the minimum population of 375 were merged with the neighbouring Data Zone with the longest shared boundary within the same Intermediate Zone. Conversely, Data Zones which had seen an increase in population to values greater than 1,125 (e.g. as the result of a new housing development) were split into one or more new Data Zones.

After the split and merge process was complete, additional changes were applied to reconfigure any multi-extent Data Zones which were not caused by water. This occurred where Census Output Areas had changed significantly from 2011 and resulted in some best-fit 2022 Data Zones being composed of two parts that do not touch. In most instances, the smaller non-contiguous part of the Data Zone (usually consisting of a single Output Area) was absorbed into a neighbouring Data Zone. In addition, we had already received feedback from some local authorities on changes to make to Data Zone boundaries, which was considered and applied where possible when designing the proposed 2022 Data Zones.

A similar process was followed for Intermediate Zones. The first step was to best-fit the proposed 2022 Data Zones onto the 2011 Intermediate Zone boundaries using the Data Zone Population Weighted Centroids. Then, any ‘best-fit’ 2022 Intermediate Zones which fell below the population threshold of 2,125 household residents, or above 6,375 household residents, were reconfigured. In addition, any best-fit 2022 Intermediate Zones which had become multi-extent (excluding those caused by islands) were reconfigured.

Centroids

Once the proposed boundaries had been created, centroids were calculated. This is not the geometric middle of the Data Zone or Intermediate Zone, but a point that represents the population weighted centre. The main use of a centroid is to allocate a Data Zone or Intermediate Zone to a higher-level geography. The ‘median method’ which was used in 2011 has also been used to calculate the proposed 2022 Data Zones and Intermediate Zones centroids. The method is as follows:

1. The median easting and northing coordinate pair for all Census Output Areas within the Data Zone is calculated, weighted by the household residents population, giving a notional centroid of the Data Zone.

2. The distance from each of the Census Output Area centroids to the median easting/northing is calculated. The Census Output Area centroid with the shortest distance to the median is chosen as the centroid of the Data Zone.

The same process is followed for Intermediate Zones, using the Data Zone centroids as the input (i.e. the Data Zone centroid closest to the median easting/northing is chosen as the Intermediate Zone centroid).

Population and household totals

The total number of households, total population, and the household residents figures are available for all proposed Data Zones and Intermediate Zones. All census data outputs have privacy protection applied (known as cell key perturbation) to keep the data of individuals safe. In the case of 'nested' geography hierarchies, the impact of these Statistical Disclosure Controls (SDC) means the sum of the population within Output Areas does not always match the corresponding Data Zones population totals.

Similarly, the sum of the population within Data Zones might not match the corresponding Intermediate Zones population totals. The same applies when summing totals at a local authority level, i.e. the sum of the populations of all Output Areas, Data Zones, and Intermediate Zones within a Local authority may not match the published Local authority total. Any new or reconfigured Data Zones or Intermediate Zones which are proposed via this consultation will be checked by the NRS Census team to ensure that the perturbed household residents totals meet the population thresholds. More information about the SDC applied to census data is available on the Scotland's Census website.

Names and Codes

We will contact local authorities to ask for names for Intermediate Zones (and local authorities can name Data Zones too, if they wish). In addition, official S-codes will be assigned using the ‘S01’ entity for Data Zones, starting at S01013482, and ‘S02’ entity for Intermediate Zones, starting at S02002515. Temporary codes have been used for the proposed Data Zones and Intermediate Zones in this consultation.