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RESEARCH PRODUCT

Spatial Vote Redistribution in Redrawn Polling Units

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Summary A large proportion of electoral analyses using geography are performed on a small area basis. In each new election there are always modifications to the previously existing polling units. The use of past voting results in small area aggregate data electoral forecasting models and political analyses therefore requires establishing a correspondence between old and new polling units. Traditionally, the task of tracking changes to assign an electoral history to the new units properly has been carried out by hand, comparing unit codes and census figures. This is an extremely cumbersome task that cannot always be performed, as when a massive (geographically intense) reorganization of polling unit boundaries takes place. Nowadays, however, assisted by the increasing availability of geographical data, this chore could be easily automated and even improved with the help of spatial statistical software. The paper suggests several methods for allocating votes by using geographical information systems tools and shows the effectiveness of spatial strategies. These approaches will permit electoral pollsters and forecasters to solve the issue efficiently and to apply the most successful electoral forecasting techniques that are currently in use and will help electoral geographers with the problem of comparing spatial aggregate electoral data from different elections. The relevance of the analysis, nevertheless, goes beyond electoral data, as the reallocation of data from one set of administrative units onto another arises in many applications. The geometric approach is proposed as a natural substitute for the classical approach and three additional approaches (centroid, surface and compositional) are also suggested, exploiting the spatial patterns that electoral outcomes display. The relative performance of the various methods is assessed in three real data instances. The results suggest that the surface approach, which obtains past voting outcomes in each polling unit by averaging their vote proportion interpolations, is the most suitable procedure.