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Census flow data or origin-destination flow statistics provide the UK academic community with a rich source of information on spatial mobility. Census flow data comprise the Special Migration Statistics (SMS) and the Special Workplace Statistics (SWS). However, in the 2001 Census, Special Travel Statistics (STS) were released for Scotland which include travel to work as well as travel to study data. Census flow data have been collected and published from 1981, potentially providing an opportunity for researchers to study spatial patterns and trends for small geographical areas over the two decades from 1981 to 2001.
However, it is not a straightforward task to compare census flow data through time. Many difficulties occur including: changes in the definition of census questions; changes in the selection of census topics; alterations to the sample coverage; and the adoption of different disclosure control methods (Stillwell & Duke-Williams, 2007). Boundary changes between censuses are also a significant problem that hampers the comparison of census data though time (Gregory and Ell, 2005; Champion, 1995). At a relatively small area scale, wards are a commonly used set of geographical units for analysing census data, but they change frequently because they are electoral areas and boundary changes are inevitable to satisfy electoral equality. As a consequence, between 1981 and 2001 wards in Britain have experienced constant review and adjustment by the Boundary Commissions of local governments (Rees, 1995). Our calculations based on the ONS Ward History Database show that about 46% of wards in England and Wales have been subject to one or more geographical boundary change in the 1990s. At a more aggregate spatial scale, major changes occurred in the mid-1990s at the local government district level with the re-organisation of local government and at the regional scale with the emergence of Government Office Regions.
The conventional response to the problem of boundary change has been to examine socio-demographic changes using larger, more consistent areal units. Thus, Boyle (1994; 1995), Champion (1994) and Champion et al. (1998) used relatively large areas to study population migration. Similarly, Frost et al. (1996; 1997; 1998) compared commuting data for 1981 and 1991, but were forced to group wards into concentric bands from metropolitan centres, or to summarise flows for entire inner cities. These approaches have limitations because, on the one hand, most people tend to move or commute over short distances and, on the other hand, research at more aggregate levels may fail to reveal the true factors underlying these flow patterns. Hence the need to develop a suitable methodology to estimate interaction flows from censuses for spatially consistent zones so that they could be compared both spatially and temporally.
This paper is organised as follows. First, we introduce the characteristics of the SMS and SWS for 1981, 1991 and 2001, then we describe why estimating flow data for different geographies needs a novel methodology. In the fourth section we summarise the main stages of the estimation strategy and explain how we solved the various problems associated with integrating interaction data through time. Finally we demonstrate two separate examples involving the investigation of migration change in England and commuting patterns in Birmingham to demonstrate the use of the estimated migration and commuting flow data.