is explicitly claimed public by a peer", and they define such attributes as "claimed attributes". The third category of interests is group attributes, defined as "location or affiliation oriented" and are needed to form a...basis for communities", an example being the "domain name of an internet connection" which acts as an online location and group identifier for certain users. Cost reduction Cost reduction influences the sharing component of P2P communities. Users who share do so to attempt "to reduce...costs" as made clear by Cunningham, Alexander and Adilov. In their work Peer-to-peer File Sharing Communities, they explain that "the act of sharing is costly since any download from a sharer implies that the sharer is sacrificing bandwidth". As sharing represents the basis of P2P communities, such as Napster, and without it "the network collapses", users share despite its costs in order to attempt to lower their own costs, particularly those associated with searching, and with the congestion of internet servers. User motivation and size of community User motivation and the size of the P2P community contribute to its sustainability and activity. In her work Motivating Participation in Peer to Peer Communities, Vassileva studies these two aspects through an experiment carried out in the University of Saskatchewan (Canada), where a P2P application (COMUTELLA) was created and distributed among students. In her view, motivation is "a crucial factor" in encouraging users to participate in an online P2P community, particularly because the "lack of a critical mass of active users" in the form of a community will not allow for a P2P sharing to function properly. Usefulness is a valued aspect by users when joining a P2P community. The specific P2P system must be perceived as "useful" by the user and must be able to fulfil his or her needs and pursue his or her interests. Consequently, the "size of the community of users defines the level of usefulness" and "the value of the system determines the number of users". This two way process is defined by Vassileva as a feedback loop, and has allowed for the birth of file-sharing systems like Napster and KaZaA. However, in her research Vassileva has also found that "incentives are needed for the users in the beginning", particularly for motivating and getting users into the habit of staying online. This can be done, for example, by providing the system with a wide amount of resources or by having