Introduction

Membership of an organization or participation in an event is a source of social ties. In organizations and events, people gather because they have similar tasks or interests and they are likely to interact. Members of a sports club, for instance, share a preference for a particular sport and play with or against one another. Directors and commissioners on the boards of a corporation are collectively responsible for its financial success and meet regularly to discuss business matters. Inspired by the sociology of Georg Simmel, groups of people that gather around one or more organizations and events are called social circles.

In previous chapters, we studied direct ties among people, such as the choice of friends, or among other social entities, for instance, trade relations between countries. Note that we studied relations among actors of one kind: relations between people or between organizations but not between people and organizations. Now, we focus on the latter type, which is called an affiliation. Data on affiliations can be obtained relatively easily, and they are very popular in data mining.