June 6, 2008
Nature magazine cover story discusses revolutionary findings
In a groundbreaking paper published as a cover story in this week’s Nature magazine, Northeastern University physicist Professor Albert-László Barabási and his team found that humans can be characterized based on how they move. In the article, titled “Understanding Individual Human Mobility Patterns,” the authors discuss how, for the first time, they were able to follow individuals in real-time and discovered that despite the diversity of their travel history, humans follow simple reproducible patterns.
Barabási, along with co-authors Marta C. González and César A. Hidalgo, studied the trajectory of 100,000 anonymized cell phone users – randomly selected from more than 6 million users – and tracked them for a six-month period. They found that contrary to what the prevailing Lévy flight and random walk models suggest, human trajectories show that while most individuals travel only short distances and a few regularly move over hundreds of miles, they all follow a simple pattern regardless of time and distance, and they have a strong tendency to return to locations they visited before.
“We found that human trajectories show a high degree of temporal and spatial regularity, each individual being characterized by a time-independent characteristic travel distance and a significant probability to return to a few highly frequented locations, like home and work” said Albert-László Barabási, Distinguished Professor of Physics and Director of the Center for Complex Network Research (CCNR) at Northeastern University.
“Our study shows that humans, after only three months of saturated behavior, reach stability in their mobility patterns, and the trajectories become identical,” added Marta C. González, Ph.D. in Physics and Research Assistant at the CCNR. “People devote their time to a few locations, although spending their remaining time in five to 50 places, visited with diminished regularity.”
The location of cell phone users was located every time they received or initiated a call or a text message, allowing Barabási and his team to reconstruct the user’s time-resolved trajectory. In order to make sure that the findings were not affected by an irregular call pattern, the researchers also studied the data set that captured the location of 206 cell phone users, recorded every two hours for an entire week. The two data sets showed similar results, the second validating the first.
The findings of this research complement the notion that human mobility can be generalized by the Lévy flight statistics, as suggested by a 2006 study that found that bank note dispersal is a proxy for human movement. That study analyzed the dispersal of about half-a-million dollar bills in the U.S. and concluded that human travel on geographical scales is an ambivalent and effectively superdiffusive process. By using a different methodology, Barabási’s group was able to find evidence to complement those findings.
“Contrary to bank notes, mobile phones are carried by the same individual during his/her daily routine, offering the best proxy to capture individual human trajectories, said César A. Hidalgo, Ph.D. and Research Assistant at the CCNR. “Also, unlike dollar bills that always follow the trajectory of the current owner and diffuse, humans display significant regularity and do not diffuse.”
“The inherent similarity in travel patterns of individuals could impact all phenomena driven by human mobility, from epidemic prevention to emergency response, urban planning, traffic forecasting and agent-based modeling,” added Barabási.
The study relied on a sample from anonymized, aggregate billing data from cell-phone users in an unidentified European country. The Institutional Review Board at the U.S. Office of Naval Research, which funded this study as part of a larger pool of research into human mobility patterns, reviewed the proposal in June 2007 and determined that it did not involve human subjects.
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Founded in 1898, Northeastern University is a private research university located in the heart of Boston. Northeastern is a leader in interdisciplinary research, urban engagement, and the integration of classroom learning with real-world experience. The university's distinctive cooperative education program, where students alternate semesters of full-time study with semesters of paid work in fields relevant to their professional interests and major, is one of the largest and most innovative in the world. The University offers a comprehensive range of undergraduate and graduate programs leading to degrees through the doctorate in six undergraduate colleges, eight graduate schools, and two part-time divisions. For more information, please visit www.northeastern.edu.