Real-World and Virtual Communities

There are various notions of what constitutes a community. According to the Oxford English Dictionary, the term community has its origin in the Old French word comunete which again has its origin in the Latin word communis meaning “fellowship, community of relations or feelings”. In medieval Latin it was, like universitas, used concretely in the sense of “a body of fellows or fellow-townsmen”.

Definition of the Term “Community”

The Merriam-Webster Online Dictionary offers several definitions for the term “commu- nity” such as “the people with common interests living in a particular area”, “an interacting population of various kinds of individuals (as species) in a common location”. These examples stress two important and typical characteristics: (i) common interest and (ii) interacting individuals.3

Hillery [87] found through a meta-analysis of 94 definitions of the term community that 69 definitions included the following aspects: social interaction, common ties and a geographic criterion as characteristics of the concept. The geographic aspect attributes

to the fact that it has been observed that people’s lives were intertwined with the local institutions and that a complex network of people, institutions, shared interests, locality and a sense of psychological “belonging” had been identified. The community concept originates from this observation [160]. In other words, people come together, physically, or by other means, because they have something in common, a shared purpose, and this results in shared activities and social interactions.

Depending on the context, the noun community has several meanings such as:

• a group of people whose members reside in a specific locality, share government, and often have a common cultural and historical heritage (see, e.g., [163])

• a locality inhabited by such a group (see, e.g., [34, 111])

• a group of people with a common religion, race, or profession (e.g., the scientific community [126])

• a group of people sharing certain attitudes and interests in common (community of interest) (see, e.g., [159, 94, 137])

• a group of people who share a concern, a set of problems or a passion about a topic and who deepen their knowledge and expertise by interacting on an ongoing basis (community of practice) (see, e.g., [166])

• a group of associated nations sharing common interests or a common heritage (e.g., the Western European Community [21])

• a group of interdependent plants or animals growing or living together or occupying a specified habitat (see, e.g, [107])

Communities in the Virtual World

Even though originally the term community was used for groups of individuals living or interacting in a physical environment, it is nowadays often also applied to groups of people interacting virtually. In the beginning of this transition, authors used the term online community or any of its synonyms such as virtual community, mediated community, electronic community, cyber community or Internet community to distinguish from physical communities.

Howard Rheingold is credited with inventing the term virtual community. In 1993, he published a book called “The Virtual Community” [139] in which he describes the cultural and political implications of a new communication medium - a computer conferencing system called the WELL (Whole Earth ’Lectronic Link). The WELL was founded in 1985 and its goal was to attract interested people into online conversation with each other. In his book he defined a virtual community as follows: “Virtual communities are social aggregations that emerge from the Net when enough people carry on those public discussions long enough, with sufficient human feeling, to form webs of personal relationships in cyberspace.” [139]. Rheingold uses the term Net for loosely interconnected computer networks that use computer-mediated communication technology to link people

2.2 Communities in Networks

around the world into discussions. He was one of the first people, if not the first, discussing his own experiences and observations with relationships being build virtually.

Later on, Wellman and Gulia elaborated in [165] on whether on-line relationships are comparable to those established in “real life”. One out of seven questions they discuss is “Are Virtual Communities Real Communities” and answer the question by putting together observations and anecdotes. The authors assess that the Net maintains strong, supportive community ties especially between people who are physically dispersed. They notice that online relationships are based more on shared interest than on shared social characteristics. An interesting observation is that while online relationships may be specialized, a virtual community as a set of ties, provides a wide range of support. Even though the question remains in the end unanswered, besides particularities that have been observed, in several aspects, virtual communities obviously have strong similarities with “real world” communities.

A precise definition of what a “community” really is does not exist yet but the usage of the term “online community” does not differ significantly from that of a physical community. More recently, Preece described an online community as a structure that consists of “people who interact socially as they strive to satisfy their own needs or perform special roles; a shared purpose that provides a reason for the community; policies that guide people’s interactions; and computer systems to support and mediate social interaction and facilitate a sense of togetherness” [137].

Preece accommodates the lack of a specific geographical area by expanding the com- munity definition with a technological communication infrastructure capable of enabling and supporting the community. Since the technological support to facilitate the social interaction is indispensable for online communities, the community provider must offer an appropriate infrastructure whose features will depend on the type of community, its (prospective) participants and its purpose.

In the computer science “community”, one of the most widely accepted and used definition is that given by Newman and Girvan in [128]: “A community is a subgraph containing nodes which are more densely linked to each other than to the rest of the graph or, equivalently, a graph has a community structure if the number of links into any subgraph is higher than the number of links between those subgraphs.” The authors assume that the links between actors are the representation of the people’s interaction. This definition goes back to the description of a cohesive subgroup by Seidman who defined cohesive subgroups as “...sets of individuals more closely tied to each other than to outsiders” [150].

In computer science the term community is also often applied to networks that represent “interactions” between computers or Web sites. For example, Kumar et al. view communi- ties as a dense bipartite subgraph of the Web graph built upon hyperlinks [110].4 Gibson et al. analyze the link topology of the WWW and view communities as structures consisting of a core of central “authoritative” pages linked together by “hub” pages [72]. Flake et al. define a Web community as a set of Web pages that link (in either direction) to more Web pages in the community than to pages outside of the community [61]. Adamic and Adar 4_{A bipartite graph is a graph whose set of vertices can be split into two subsets A and B in such a way}

use hyperlinks and information on a homepage as reflections of social interactions a user has in the real world [2]. The information such as common in- or out-links is used to predict relationships between individuals. Park follows a similar approach and interprets the social or communication structure among social actors based on the hyperlink structure among Web sites [134]. Even though the detection of communities was not the main aim, the application of social network analysis methods to study hyperlink networks was proposed already in the late nineties by Jackson [95]. The idea was to analyze the communication structure between individuals based on the hyperlink network in the WWW.

As we have seen, the term community is used in many disciplines and the definitive driver for a community is that all individuals have something in common. We also observe that the assessment whether this communality is fulfilled is done based on a static view of a graph. In the presented approaches, the observer determines whether a community exists or not only based on a snapshot of interactions taken at a certain point in time. An aspect that is rarely discussed in the literature is how a community relates to temporal aspects.

In this thesis, this traditional static view on communities is extended to capture tempo- ral evolution. We investigate communities over time taking two perspectives which allow as to capture different aspects of community dynamics:

1. We define a community as a group of individuals observed at different time points. 2. We define a community as a group of individuals that evolves over time.

A community mining approach to detect communities as objects that can be observed over time is presented in Chapter 3. There, a community is defined as a constellation that is observable over time. This constellation consists of sets of similar subgroups obtained by a hierarchical clustering procedure. In Chapter 4, an approach to detect and observe communities defined as groups of people that evolve over time is discussed.