In this chapter we discuss methods for analyzing a special kind of twomode social network that represents the affiliation of a set of actors with a set of social occasions (or events). We will refer to these data as affiliation network data, or measurements on an affiliation variable. This kind of two-mode network has also been called a membership network (Breiger 1974, 1990a) or hypernetwork (McPherson 1982), and the affiliation relation has also been referred to as an involvement relation (Freeman and White 1993).

Affiliation Networks

Affiliation networks differ in several important ways from the types of social networks we have discussed so far. First, affiliation networks are two-mode networks, consisting of a set of actors and a set of events. Second, affiliation networks describe collections of actors rather than simply ties between pairs of actors. Both of these features of affiliation networks make their analysis and interpretation somewhat distinct from the analysis and interpretation of one-mode networks, and lead us to the special set of methods discussed in this chapter. Among the important properties of affiliation networks that require special methods and interpretations are:

• Affiliation networks are two-mode networks

• Affiliation networks consist of subsets of actors, rather than simply pairs of actors

• Connections among members of one of the modes are based on linkages established through the second mode

• Affiliation networks allow one to study the dual perspectives of the actors and the events

We will return to these ideas throughout this chapter.