Consultation regarding the redraw of Data Zones

9. Changes to Census Output Areas

9.1. The process of creating boundaries for output areas for the 2011 Census was largely automated. Each postcode in the underlying postcode geography at the point when it is frozen for the purposes of the 2011 Census enumeration (January 2011) where allocated initially to its 2001 Census Output Area. Some COAs have been merged with a neighbouring COA (e.g. where housing demolition means their population has dipped below confidentiality thresholds, causing a disclosure risk), and some have been split (e.g. where new housing has taken them above maximum population thresholds.) This approach is designed to ensure as much comparability as possible between the 2001 and 2011 output geographies.

9.2. 2011 postcodes have been allocated to 2001 COA boundaries on a best-fit basis. These aggregations have formed the first cut of 2011 COAs. The 2011 COAs were then aligned to Local Authority (LA) boundaries, as some postcodes cross LA boundaries and some LA boundaries have changed since 2001. COAs outwith the existing population ranges have been split into 2 or more COAs or merged with a neighbouring COA to ensure a roughly standard population across all COAs. Where a split or merge has occurred, it has been made along a locally recognised boundary. A hierarchy of boundaries has been used to determine where the new boundaries should fall where a COA is being split or merged. As Data Zones are built up from 2001 COAs, 2011 COAs should have a good level of comparability with 2001 Data Zones.