Introduction

In this chapter, we present the concepts of centrality and centralization, which are two of the oldest concepts in network analysis. Most social networks contain people or organizations that are central. Because of their position, they have better access to information and better opportunities to spread information. This is known as the ego-centered approach to centrality. Viewed from a sociocentered perspective, the network as a whole is more or less centralized. Note that we use centrality to refer to positions of individual vertices within the network, whereas we use centralization to characterize an entire network. A network is highly centralized if there is a clear boundary between the center and the periphery. In a highly centralized network, information spreads easily but the center is indispensable for the transmission of information.

In this chapter, we discuss several ways of measuring the centrality of vertices and the centralization of networks. We confine our discussion of centrality to undirected networks because we assume that information may be exchanged both ways between people or organizations that are linked by a tie. Concepts related to importance in directed networks, notably prestige, are discussed in Part IV of this book.