Achieving Data Compatibility Over Space and Time: Creating Consistent Geographical Zones

Geographers have long-recognised the importance of boundary specification and the problems of using arbitrarily defined areas for the collection and dissemination of socioeconomic data. The focus has tended to be on the modifiable areal unit problem and on custom zone design with the problems created by temporal inconsistencies in zonal boundaries having less consideration. This is surprising as alongside occasional major structural reorganisations the UK experiences frequent administrative boundary changes causing difficulties in producing comparable statistics over time. Unless a consistent geographical approach with time-series data is taken it cannot be known whether changes are real or an artefact of boundary changes. The late 1990s has seen initiatives from ONS to promote harmonisation of geographical information and the Update UK Area Masterfiles (UUKAM) project which allows the conversion of data between 1991 census and various late-1990s geographies. For studies which predate the ONS initiative and exceed the data conversions possible through the UUKAM project, a method must be devised to establish a data time-series on a consistent geographical basis otherwise the data quality will be compromised and analyses cannot objectively be compared over time. After illustrating the nature of the boundary change problems to be overcome, this paper describes and appraises methods by which data can be adjusted to an appropriate geography. The paper concludes with a list of advised checks for researchers carrying out similar work.