Standard modularity is unsuitable for functional regionalisation of spatial interaction data

Abstract

Functional regions capturing local socioeconomic dynamics are a framework for territorial statistics and policymaking. The usefulness of such statistics and policies depends on the adequateness of the regions used in the analysis. Several authors have adopted the modularity quality function to drive their regionalisation methods. This function, originally devised for non-spatial data, has limitations particularly relevant in this context. The paper discusses these limitations and illustrate them using real data for the United States. The results warn against using standard modularity to assess or optimise the quality of a functional delineation, and advocate for available alternatives that consider the spatial nature of functional regionalisation.