How sensitive are measures of polycentricity to the choice of ‘centres’? A methodological and empirical exploration

In the literature dealing with measuring ‘urban polycentricity’, relatively little attention has been paid to the identification of what constitutes an ‘urban centre’. In this paper, we assess the sensitivity of polycentricity measures to one particular aspect of this identification: using the case of ‘polycentric urban regions’, we empirically examine the sensitivity of the ‘level’ of polycentricity to the number of cities included in the analysis. Using a two-mode firm–city data source, we do so by stepwise measuring the polycentricity of the Yangtze River Delta (YRD) as cities are added to the analysis. This measurement of ‘stepwise polycentricity’ is benchmarked against an ideal typical urban system with a rank-size distribution. The results suggest that the measure of polycentricity is indeed highly sensitive to the choice of the number of cities, so that the alleged level of polycentricity is contingent on the choice of a cut-off point. We propose that the analysis of the sensitivity of polycentricity can help researchers to (1) investigate the different role of cities in shaping polycentric structures of urban regions, and (2) better identify mono- or polycentric structures of urban regions. Analyses of trends in the ‘stepwise polycentricity’ of the YRD and seven other urban regions along the Yangtze River Economic Belt serve to illustrate.