"The structure of those networks can tell you quite a lot about how the systems work, but they're far too big to analyze by just putting dots on a piece of paper and drawing lines to connect them," said Mark Newman, an assistant professor of physics and complex systems at the University of Michigan.
One challenge in making sense of a large network is finding clumps---or communities---of members that have something in common, such as Web pages that are all about the same topic, people that socialize together or animals that eat the same kind of food. Newman and collaborator Michelle Girvan, a postdoctoral fellow at the Santa Fe Institute in Santa Fe, New Mexico, have developed a new method for finding communities that reveals a lot about the structure of large, complex networks. Newman will discuss the method and its applications Feb. 15 at the annual meeting of the American Association for the Advancement of Science in Seattle.
"The way most people have approached the problem is to look for the clumps themselves---to look for things that are joined together strongly," said Newman. "We decided to approach it from the other end," by searching out and then eliminating the links that join clumps together. "When we remove those from the network, what we're left with is the clumps."
The researchers tested their method on several networks for which the structure was already known---college football conferences, for example. In college football, teams in the same conference face off more frequently than teams in different conferences. When inter-conference games do occur, they're more likely to be between teams that are geographically close together than between teams that are far apart. Plugging in information on frequency of games between pairs of teams in the 2000 regular season, Newman and Girvan tested their method to see if it could correctly sort the colleges into conferences. "There were a few cases where it made mistakes, but it got well over 90 percent of them right," said Newman. "It gave us the structure we were expecting, so that was encouraging."
Newman and Girvan---and other researchers who've learned about their work---have gone on to apply the technique to systems where the structure is not as well understood, looking at everything from networks of Spanish language web logs to communities of early jazz musicians to a food web of marine organisms living in Chesapeake Bay.
"Networks and other systems that we study are becoming increasingly large and complicated these days," said Newman. "New methods like this help us to make sense of what we see and to understand better how things work."