Exploring the implications of changing census output geographies for the measurement of residential segregation: the example of Northern Ireland 1991–2001

One problem in analysing social and demographic change through time using census data arises from differences between censuses in the size and shape of the geographical units used to output data. Failure to correct for changing output geographies may lead to unknown and possibly large biases when comparing different censuses. The paper addresses this issue using the example of residential segregation in Northern Ireland (NI). It has two main objectives. Firstly, by recompiling 2001 NI census data on 1991 census output geographies it assesses the sensitivity of indices of residential segregation to these changes in geographical units. Secondly, it suggests a method by which census analysts can assess how sensitive their results are to changing output geographies when they are unable to correct for these changes and must work with the data ‘as they are’. A subsidiary aim is to contribute to the evidence base on residential segregation in NI. The paper finds that indices of residential segregation are insensitive to changes in output geographies between 1991 and 2001. The reason suggested for this is that the units in each zonal geography are smaller than the spatial scale over which population counts are positively autocorrelated. The use of spatially-weighted segregation indices is advanced as a generalisable means of learning about the geographical patterning of population in different censuses. It is argued that these insights combined with knowledge of the size of geographical units used in each census can help researchers elsewhere judge how sensitive their results might be changing census output geographies through time.