Many centrality indices are based on shortest paths, measuring, for example, the average distance from other vertices (Brandes and Pich (2006) Int. J. Bifur. & Chaos 17, 7). Simmons and Jones (2003) Prog. Plan. 60, 1 use it as a measure of the external market served by a city, and Channell and Lomolino (2000) Nature 403 use it to distinguish between ‘central species’ and ‘peripheral species’. See Beauchamp (1965) Behav. Sci. 10 for an ‘improved index of centrality’.