Consistent Areas Through Time (CATTs)

Researchers regularly need to standardise their data to make reliable comparisons. However, standardising geographical units is often overlooked, possibly because users are unaware of subtle changes in the configuration of geographical areas (e.g. Census output areas). There are a number of methods available to create standardised, or consistent, geographies, but most methods require the use of areal interpolation techniques, which inevitably incur error – especially when the population is not evenly distributed across a zone. Moreover, there are very few examples of consistent geographies based on small areas.

As part of his PhD research at University of St Andrews, Daniel Exeter developed a geographical boundary file, known as Consistent Areas Through Time, (CATTs), that enables reliable comparisons of social data from 1981, 1991 and 2001 census data. Rather than using an apportioning or areal interpolation technique to create a consistent geography, a merging strategy was used, which meant that whenever a source zone (e.g. 1991 OAs) overlapped more than one target zone (e.g. 1981 EDs), the affected target zones were merged.

The merging process linked the 17,767 1981 EDs, 38,254 1991 OAs and the 42,604 2001 OAs onto one common boundary file containing 10,058 CATTs.

More information regarding the construction of the CATTs can be found from the Office for National Statistics (ONS) population trends article The Creation of Consistent Areas Through Time (CATTs) in Scotland, 1981-2001.

Recently (January 2019), Dan and his colleagues updated the CATTs, to incorporate the 2011 OAs. In order to align with the 2011 geographies, they used the same merging approach to first combine the 2001 OAs with the 2011 OAs into “SUPER OUTPUT AREAs”. Next they integrated the SUPER OAs with the SUPER EDs (which combined the 1981 EDs with 1991 OAs) to create 8,548 CATTs linking 1981 EDs with the 1991, 2001 and 2011 OAs into a common file.

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A list of common definitions and acronyms used in these pages is available on our Glossary page.