Wednesday, 24 February 2010

How Predictable is our Movement?

Physicist Albert-László Barabási, well known for his work on network theory, has tuned his attention in a recent paper to the human movement. In the latest issue of Science 19 February 2010
Vol 327, Issue 5968, his paper 'Limits of Predictability in Human Mobility' reports the research work undertaken with 50'000 anonymized mobile phone user data.
Barabási has don a lot of work on networks as early as 1999 were he coined the term Scale Free Networks, describing a type of networks with major hubs, such as for example the world wide web. In his barabasilab at Northeastern University, Centre for Complex Network Research a number of network related project are researched.

Image taken from The University of Chicago / Diagram of a scale-free network that contains components with a highly diverse level of connectivity. Some components form highly interconnected hubs, while other components have few connections, and there are many levels of interconnectivity in between.

However in this recent work the focus is on the predictability of human movement. The authors say: "By measuring the entropy of each individual's trajectory, we find a 93% potential predictability in user mobility across the whole base. Despite the significant differences in the travel patterns, we find a remarkable lack of variability." The work was intended to close a gap in the approaches to modeling human behavior. Despite personally we rarely perceive our actions as random, the existing models are largely based on the factors of random movement. The paper demonstrated that even though the activities, distances and motivations for individual movement might be very divers and different the predictability of an individuals location is not. They all have very similar predictability values, ranging between 80 % and 92 %. AOL News titles their article on the work "Study Makes It Official: People Are So Predictable" implying that this must be soooo boring.

Image taken from AOL News / These diagrams represent the movements of two mobile phone users. The one on the left shows that the person moved between 22 different cell towers during a three-month period, and placed 52 percent of his calls from one area; the other subject hit 76 spots, and was much less rooted.

This might be very surprising news for most people. The fact that there is so much less changing and spontaneity might seem unrealistic, but a similar impression was given by the data collected with the UrbanDiary project last year. Even though this was a really small sample, the fact that individuals travel most of the time along their known routes, between only a few hot spots clearly emerged. This can also be seen visualised in the What Shape are You? renders. Also Hagerstand's work pointed in to this direction arguing that the 'Constraints' are too strong for too many out of rhythm activities.
Barabási already undertook similar work with mobile phone data in 2008, which war published as an article in nature, by Gonzalez MC, Hidalgo CA, Barabasi A-L. with the title 'Understanding individual human mobility patterns'. In this article they analysed data of 100'000 mobile phones. Was the media coverage back then (two years) very much concerned about privacy issues related to the data source, for example NYTimes is this less of an issue. Nevertheless it is obvious that the researchers try to play it save by mentioning about ten times in the article that they work with anonymized data.
The argument is largely the same in both articles and the finding too. In both papers the researchers show their surprise about the outcome, that the movement can be predicted. However to my surprise they stick to their study and do not draw any strong links to routines and rhythms of personal habits. You can listen to a podcast where Barabási talks about this research.
In the more recent paper they conclude "At a more fundamental level, they also indicate that, despite our deep-rooted desire for change and spontaneity, our daily mobility is, in fact, characterized by a deep-rooted regularity."
I believe that the former, spontaneity, is very much a cultural phenomenon similar to the urge to stay young. The later, regularity, is the provider of identity and orientation resulting in stability and safety and therefor fundamental to human everyday life. Interesting should be Barabási's upcomming new book Burst on "The Hidden Patterns Behind Everything We Do".

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